Why is my car making a knocking noise?

A knock is the noise people put off longest, because a car that knocks usually still drives. Nothing is flashing, nothing is smoking, and the noise often fades once you are up to speed, so it gets filed under things to sort out later. In one specific case that instinct is badly wrong, and in the others it is only roughly right. This guide is about telling which is which, before you pay anyone.

The first thing worth knowing is that the word knocking covers two problems with nothing in common. One is a worn part in the suspension, the brakes or the driveline, which gets slowly worse and gets more expensive the longer it is left. The other is combustion knock inside the engine, which is not a broken part at all and can sometimes be cured by the next tank of fuel. They happen at different moments, they mean different things, and there is a free test that separates them in about thirty seconds.

The thirty second test that halves the list

Before you try to describe the noise to anyone, find out whether it follows the engine or the wheels. Every knock belongs to one group or the other, and the test needs nothing but your driveway.

Park on level ground, apply the handbrake, select neutral and let the engine idle. Now blip the throttle gently, up to around 2,000rpm and back, a few times. The car is not moving, so nothing in the suspension, the brakes or the driveshafts can make a sound. If the knock appears and quickens with the revs, it belongs to the engine or to something bolted to it. If it stays silent, the noise needs the wheels to be turning.

The second half of the test sorts out that group. On a quiet road at about 30mph, with nothing behind you, slip into neutral and coast. The wheels are still turning but the engine is no longer driving them. A knock that carries on while you coast is almost certainly suspension, a wheel bearing or a brake component. A knock that disappears the instant you lift off and returns the moment you accelerate is pointing at the mountings or the driveline.

Those two tests put you in the right half of the list before a garage has touched the car. It is the difference between arriving with a knocking noise and arriving with a knock that follows road speed and carries on when I coast in neutral. The second version saves the technician a road test, and a road test is exactly what you are paying for.

Knocking over bumps: the suspension

A knock or clunk as you cross a speed bump, a pothole or a drain cover is the most common knock of all, and it means a joint somewhere has enough free play to move and hit its own stop. The usual suspects, in rough order of how often they turn out to be the answer, are the anti-roll bar drop links, the rubber bushes in the suspension arms, a worn strut top mount, and a shock absorber that has lost its damping.

Drop links are worth knowing about because they are among the cheapest parts on a car and they are a very common cause of exactly this noise. A car that clonks over every bump and drives perfectly well otherwise very often needs a pair of links and nothing else, which is a genuinely small bill. That is also why it is worth having the noise identified rather than assumed: the fear is a big suspension job, and the reality is frequently not.

What makes this worth acting on is that the MOT does not treat suspension play as a matter of opinion. The inspection manual gives testers a measurable limit for wear in suspension joints, pins and bushes: play is excessive if it is more than 2mm on a 12mm diameter pin, 3mm on a 25mm pin, or 10% of the pin diameter on anything larger than that. Shock absorbers get their own rules, and one with negligible damping effect is a Major defect, as is a shock absorber bush that is excessively worn. So a knock over bumps is not only a noise, it is a countdown to a failed test, and the same wear also shows up as vague steering and a longer stopping distance long before it fails.

If the knocking comes with a wobble through the wheel, read our guide to a shaking steering wheel, which covers the overlap between suspension wear, wheel balance and warped discs. For what the repairs actually cost, our guide to suspension repair costs sets out realistic UK prices, from around £80 for a simple bush to £500 or more per corner for shocks or springs. And if the last test wrote something up rather than failing it, our guide to MOT advisories explains what that actually obliges you to do.

Knocking or grinding when you brake

If the noise only appears as you press the brake pedal, look at the brakes. Pads at the very end of their life, discs that are badly worn, a calliper sticking on its slide pins or a missing anti-rattle clip will all let the pad move in its carrier and knock. A deeper grinding note usually means the friction material has gone entirely and metal is running on metal, which damages the disc as well as the pad.

This is the knock with the least room for a wait and see approach, because the MOT grades it severely. A brake lining or pad worn down to its wear indicator is a Major defect. One worn below 1.5mm is Dangerous, which is the category the manual reserves for a defect that is an immediate risk to road safety. A brake disc that is significantly and obviously worn is a Major, and one that is insecure or fractured is Dangerous. Our guide to brake warning signs you should not ignore covers the noises and pedal feelings that mean now rather than soon, and what new brakes cost gives the prices so you can judge a quote.

One useful distinction: a squeal is usually the wear indicator doing its job, a light tapping tick that changes with road speed and stops when you brake is often a stone caught in the disc shield, and a knock that is felt through the pedal as well as heard points at a disc problem rather than a pad problem.

A click or knock on full lock: the driveshaft joint

A rhythmic clicking or knocking that appears only on tight, slow turns, the kind of thing you hear reversing out of a parking space with the wheel on full lock, is the classic signature of a worn constant velocity joint on a driveshaft. It is one of the few car noises that is close to diagnostic on its own.

The part most people miss is why the joint wore out. A CV joint is packed with grease and sealed inside a ribbed rubber boot. The boot splits, usually from age and road salt rather than impact, the grease is thrown out, grit gets in, and the joint then destroys itself over the following months. So the boot is the cause and the clicking joint is the effect, which matters because the two are priced very differently. A split boot caught early is a modest job. A joint that has already been running dry means a replacement driveshaft, and what a CV joint replacement costs puts real numbers on both ends of that.

The MOT reflects exactly that sequence. Under the transmission section of the inspection manual, a CV joint boot that is severely deteriorated is a Minor defect, but a boot that is missing, split or insecure so that it no longer prevents the ingress of dirt is a Major. And a transmission joint that is excessively worn is a Major in its own right. In other words, the test gives you one grade of warning while the fix is still cheap, then fails you once the damage is done. If you have ever had a boot mentioned on a test result and left it, that is what the note was for. You can look back through your own history for free on the DVSA MOT history service.

A clunk as you pull away or change gear

A single clunk as you take up drive, change gear, or lift off and get back on the power is a different family again. The most common cause is a worn engine or gearbox mounting. Mountings are rubber and metal blocks that hold the engine while absorbing its movement, and when the rubber perishes or separates the whole drivetrain rocks further than it should and comes up against something.

This is another one the MOT tests directly. An engine mounting or bracket that is severely damaged or deteriorated resulting in excessive movement is a Major defect, and one that is fractured, missing or excessively loose is Dangerous. That is worth knowing, because a mounting clunk is easy to live with for months and sounds far less alarming than it is graded.

Other candidates in this group are wear in the driveshaft splines, play in the differential, and on a manual car a worn dual mass flywheel, which typically also rattles at idle and settles when you press the clutch. If the clunk arrives with a slipping or juddering take up, our guide to clutch replacement costs and the one on gearbox replacement will tell you what sort of numbers you are looking at, so you can judge whether a quote is fair.

A knock from the engine that rises with the revs

This is the knock that changes what you should do next. A deep, heavy, rhythmic knock that comes from the engine itself, is there when the car is standing still, and rises and falls directly with the revs, is the one that means stop. It is the sound of significant internal wear, most often in the bottom end of the engine, and every additional mile driven on it makes the outcome worse.

Do one thing before you assume the worst. Let the engine cool, check the oil level on the dipstick and look at the condition of the oil while you are there. Running low on oil is a common and entirely recoverable cause of a bottom end knock in its early stages, and it is also the cheapest possible explanation. Our guide to checking your engine oil covers reading the dipstick properly, and what an oil change costs explains why the grade in the handbook matters more than the price on the bottle.

If the level is correct and the knock is still there, do not drive it to the garage. Have it recovered. A tow costs a fraction of what an extra thirty miles can turn a repairable engine into. Treat it as urgent without qualification if the knock arrives alongside any of these: a warning light, especially the oil pressure light, which our guide to the engine management light puts in context, a sudden loss of power, a burning smell, smoke of any colour from the exhaust, or the temperature gauge climbing.

One near miss worth ruling out: on an engine with a timing belt, a rhythmic noise from the front of the engine can be the belt or a tensioner rather than the bottom end, and that is a far better outcome as long as it is caught. If the belt is anywhere near its interval, our guide on when to change a cambelt and what it costs is the relevant reading.

Pinking, the knock that is not a broken part

There is one knock that is not wear at all, and almost no article about knocking noises mentions it. Pinking, also called detonation or spark knock, is a light metallic rattle, more like marbles in a tin than a thud, that appears only when the engine is working hard. Pulling uphill in too high a gear is the classic way to provoke it. Lift off and it vanishes instantly.

What you are hearing is the fuel and air mixture igniting unevenly instead of burning in a single controlled front. The most common cause in the UK is simply the wrong grade of petrol. Standard 95 octane petrol in Great Britain became E10 in September 2021, and in Northern Ireland in November 2022, while E5 remains available in the 97 plus octane super grade. Most cars are entirely happy on E10, but a car whose handbook specifies a higher octane fuel can pink on standard petrol, and in that case a tank of super unleaded is both the test and the cure.

The other causes are carbon build up in the combustion chambers raising the effective compression, an engine running hotter than it should, or a failed knock sensor. Modern engines use a knock sensor to detect detonation and retard the ignition timing automatically, so audible pinking on a modern car often means that protection has stopped working, which is a fault worth reading. Do not simply live with it. Sustained detonation damages pistons and head gaskets, and it is one of the few knocks where the fix can cost less than a tank of fuel.

Which knocks are already an MOT failure

This is the part that turns should I bother into a decision with a date on it. Most of the parts that knock are parts the MOT inspects, and the manual grades them precisely. Here is how the common causes are categorised.

A Major or Dangerous defect is a failure. A Minor is a pass with the defect recorded, and an advisory is a note that something is on its way. Notice that the CV joint boot appears twice with two different grades, which is the test giving you a cheap warning before an expensive one. Our guide to what is checked in an MOT test walks through the rest of it, and if your test is due you can confirm the date free on the MOT check page.

When to stop driving

Most knocks are book it in this week rather than stop now. These are the exceptions.

• A deep knock that rises and falls with the revs and is present with the car stationary. Check the oil, then recover it rather than drive it.
• Any knock accompanied by the oil pressure warning light, a temperature gauge climbing, or a loss of power.
• Grinding or knocking from a wheel under braking, particularly with a longer pedal travel or a pull to one side.
• A knock that arrived at the same time as the steering going vague, or the car wandering under braking.
• A knock that changed suddenly after hitting a kerb or a deep pothole, which can mean a bent or cracked component rather than a worn one.

What it costs to find and fix

Investigation is the first spend and it is a small one. Most UK garages charge roughly £40 to £80 to trace a noise, which normally buys a road test and a proper look with the wheels off the ground. Our guide to what a car diagnostic check is explains why a plug in code read alone rarely finds a mechanical knock, since a knock usually stores no fault code at all.

Two habits keep the bill honest. Ask for the diagnosis and a written quote before any work is authorised, and ask which parts are being replaced because they are worn rather than because they are nearby. Our guide on questions to ask a garage before booking covers the wording that gets a firm answer, and the signs a garage is overcharging you covers what to watch for on the invoice.

How to describe the knock so the garage finds it first time

Noises are the hardest thing to diagnose because they are so often absent when the technician drives the car. Five specifics turn a vague complaint into something findable, and they cost you nothing but a note on your phone.

1. When it happens. Over bumps, under braking, on full lock, pulling away, or under load uphill.
2. Whether it follows the revs or the road speed. The thirty second test at the top of this guide gives you this in one answer, and it is the single most useful sentence you can offer.
3. Where it seems to come from. Front or rear, left or right, under the car or from the engine bay.
4. What it sounds like. A single clunk, a rhythmic click that speeds up with the wheels, a metallic rattle, or a deep thud.
5. When it started and what changed. Gradually over months, or the morning after a pothole, and whether cold or warm makes any difference.

If the noise is intermittent, offer to go out in the car with the technician and point it out. Most garages will happily take five minutes for that, and it is far cheaper than paying for a road test that finds nothing. Choosing somewhere that will do that is part of the job too, and our guide on how to choose a good local garage covers what to look for.

The bottom line

Use the free test first. Rev it in neutral, and if the knock is there with the wheels still it is the engine, which is the one group that can be urgent. If it is silent, the noise needs the wheels turning, and you are into suspension, brakes, bearings and driveshafts, which are worn parts on a clock rather than an emergency. Either way, the parts that knock are almost all parts the MOT inspects and grades, so a knock you ignore now is usually a failure you pay for later, alongside a repair that has grown in the meantime.

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