Strange noise from your car? Find what's making it

Describe a noise by what it sounds like and when it happens, and most can be narrowed to one or two parts. A clunk over bumps is usually suspension, a click on full lock is a CV joint, a hum that rises with road speed is a wheel bearing or a tyre, and a squeal from the engine bay is usually the auxiliary belt.

Pick the noise below for the likely causes, ranked, and what to do next.

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Strange noise

Is it petrol or diesel?

A hybrid counts as petrol. The likely causes are completely different between the two.

Is it safe to keep driving?

For a typical car with each symptom. These are general guidance, not a diagnosis — if you're unsure, don't drive it.

Clunking or knocking noise over bumps

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Anti-roll bar drop linkMost likely

    A knock over speed bumps at low speed, especially when one wheel goes over at a time. A cheap part, and often the whole story.

    If it's this: get it checked soon.

  2. Worn suspension arm bushesLikely

    A clunk when braking or pulling away, and steering that wanders at speed.

    If it's this: get it checked soon.

  3. Worn or leaking shock absorberPossible

    A bouncy, floaty ride, oil wetting the shock absorber body, and patchy wear across the tyre tread.

    If it's this: get it checked soon.

  4. Worn suspension ball jointPossible

    A clunk over bumps or when braking, and play when a front wheel is rocked top to bottom (hands at 12 and 6 o'clock) with the car jacked up.

    If it's this: get it checked soon.

What to do next

A knock over speed bumps is usually a cheap anti-roll bar drop link rather than anything sinister, but get the suspension looked at within days.

Clicking noise when turning

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. CV joint (drive shaft)Most likely

    Clicking on full lock when pulling away, and grease flung around the inside of the wheel from a split gaiter.

    If it's this: get it checked soon.

  2. Broken coil springPossible

    One corner sits lower than the others, and there's a loud clunk. A broken spring end can cut into a tyre — check before driving far.

    If it's this: get it checked soon.

  3. Power steering faultPossible

    Heavy steering with a steering warning light (electric), or a whine on full lock and a low fluid level (hydraulic).

    If it's this: get it checked soon.

What to do next

A clicking CV joint gets louder over months before it fails. Book it in, and don't leave it until it clunks.

Humming or droning noise that rises with speed

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Wheel bearingMost likely

    A hum or drone that rises with road speed and changes as you weave gently left and right.

    If it's this: get it checked soon.

  2. Damaged or under-inflated tyreLikely

    Check the pressures, and look for a bulge on the sidewall or a nail in the tread. A bulge means the tyre can fail — replace it before driving at speed.

    If it's this: get it checked soon.

  3. Wheel alignment outPossible

    The steering wheel sits off-centre on a straight road, and the front tyres wear on one edge.

    If it's this: not urgent.

What to do next

If the hum changes as you weave gently left and right, it's a wheel bearing. It won't fix itself — get it booked in.

Squealing from the engine bay

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Auxiliary (fan) belt slipping or wornMost likely

    A squeal on start-up or at full steering lock, and a glazed or cracked belt. If it snaps, you lose charging and often the water pump.

    If it's this: get it checked soon.

  2. Alternator not chargingPossible

    The red battery light is on while driving. With the engine running, a meter across the battery should read about 13.8–14.7V; much lower means it isn't charging.

    If it's this: get it checked soon.

  3. Power steering faultPossible

    Heavy steering with a steering warning light (electric), or a whine on full lock and a low fluid level (hydraulic).

    If it's this: get it checked soon.

What to do next

Usually the auxiliary belt. Cheap to replace, and worth doing before it snaps and takes the charging with it.

Rattle on a cold start

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Stretched timing chainMost likely

    A rattle for a few seconds after a cold start, sometimes with a cam/crank correlation code. It gets louder over time and can jump teeth if left.

    If it's this: get it checked soon.

  2. Exhaust blowing, corroded or looseLikely

    A rattle or blowing noise from underneath, louder on a cold start. If you can smell exhaust fumes inside the car, stop driving it.

    If it's this: get it checked soon.

  3. Turbocharger or a leaking boost pipeLikely

    A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.

    If it's this: get it checked soon.

What to do next

A cold-start rattle that fades is a classic timing-chain warning on some engines. Have it listened to soon — a chain that jumps can wreck the engine.

Rattle at idle that stops when the clutch is pressed

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Dual-mass flywheel or clutchMost likely

    Judder pulling away, a rattle at idle that stops when you press the clutch, or revs rising without the car speeding up.

    If it's this: get it checked soon.

  2. Worn engine or gearbox mountsLikely

    A clunk when pulling away or changing gear, and the engine rocking visibly when someone blips the throttle.

    If it's this: not urgent.

What to do next

Typical of a worn dual-mass flywheel. Not an emergency, but it's usually done with the clutch, so get a quote for both.

Rattle from underneath the car

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Exhaust blowing, corroded or looseMost likely

    A rattle or blowing noise from underneath, louder on a cold start. If you can smell exhaust fumes inside the car, stop driving it.

    If it's this: get it checked soon.

  2. Anti-roll bar drop linkPossible

    A knock over speed bumps at low speed, especially when one wheel goes over at a time. A cheap part, and often the whole story.

    If it's this: get it checked soon.

  3. Catalytic converterPossible

    Code P0420, a rattle from underneath, and emissions close to the limit at the MOT.

    If it's this: get it checked soon.

What to do next

Often a loose heat shield or exhaust mount. If you can smell fumes inside the car, get it checked straight away.

Whistling noise when accelerating

Drive carefully — book it in

You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.

Likely causes

  1. Turbocharger or a leaking boost pipeMost likely

    A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.

    If it's this: get it checked soon.

  2. Air flow sensor or a split boost hoseLikely

    A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.

    If it's this: get it checked soon.

What to do next

Check the intake hoses for a split first. A turbo that's getting louder and losing power needs looking at before it fails.

Knocking from the engine

Stop — don't drive it

If you're driving, pull over when it's safe and switch off. Don't drive it again until it's been checked — have it recovered if you need to. Carrying on risks the engine, your brakes or your steering, and recovery costs less than the damage.

Likely causes

  1. Low oil level or oil pressureMost likely

    Stop and switch off. Check the dipstick with the car on level ground after a few minutes. Driving with the oil light on can destroy an engine in minutes.

    If it's this: don't drive.

  2. Fuel injectorLikely

    A misfire or knock on one cylinder, and sometimes smoke. On a diesel a garage can do a leak-off test to find the bad one.

    If it's this: get it checked soon.

  3. Stretched timing chainLikely

    A rattle for a few seconds after a cold start, sometimes with a cam/crank correlation code. It gets louder over time and can jump teeth if left.

    If it's this: get it checked soon.

What to do next

Check the oil level straight away. A deep knock from inside the engine that rises with revs is serious — don't drive it far.

A shortlist, not a diagnosis

The ranking is the one a mechanic would give over the counter for a typical car — nobody can road-test a car over the internet, and the real cause may not be listed. Add the registration in the checker above to have it checked against that car's MOT advisories, mileage and engine. Have the car checked by a qualified mechanic before relying on it, and if you're unsure whether it's safe, don't drive it.

Common questions

What causes a clunking noise over bumps?

Most often a worn anti-roll bar drop link, which is a cheap part. Worn suspension bushes, shock absorbers and ball joints come next. If a tester flagged any of these at the car's last MOT, the checker moves it up the list when you add the registration.

Why does my car click when I turn?

A clicking or knocking on full lock while pulling away is the classic sign of a worn outer CV joint on the drive shaft, often after its rubber boot has split. It gets worse and eventually fails, so book it in.

How do I know if it's a wheel bearing?

A hum or drone that rises with road speed rather than engine speed, and often changes when you weave gently left and right. Unevenly worn tyres can sound very similar, so have both checked.

Is a rattle on a cold start serious?

It can be. A few seconds of rattle on a cold start is a known sign of a stretched timing chain on some engines, which is worth checking promptly because a chain that jumps can wreck the engine. It can also be something as harmless as a loose exhaust heat shield.

Related symptoms

Running poorly

Car running badly? Losing power, misfiring, stalling or juddering

Steering & vibration

Steering problems: pulling, shaking, heavy or vague steering

Brakes

Brake problems: squeaking, grinding, judder or a soft pedal

All symptoms: Engine management light · Other warning lights · Won't start · Exhaust smoke · Running poorly · Leaks · Brakes · Steering & vibration · Symptom checker