Most people describe a fault to a garage by making the noise. It is not a bad method — the sound, and when it happens, narrows things down faster than almost anything else. Here is the rough translation, and more importantly which ones mean stop.
Three questions do most of the work. Does it change with road speed, engine speed, or neither? Does it happen when you brake, when you steer, or all the time? Is it a rhythm or a constant? Answer those before you worry about the exact sound.
Knocking or rumbling from the engine
A deep, rhythmic knock from the engine that gets faster as the revs rise is the one to take seriously. It can indicate bottom-end wear, and continuing to drive can end the engine outright. Combined with an oil pressure warning light, stop.
A lighter metallic rattle under acceleration, sometimes called pinking, is a different thing and usually fuel or ignition related. Less urgent, still worth investigating.
A rattle for a second or two on a cold start that then disappears is common on some engines and worth mentioning at the next service rather than panicking about.
If the knocking turns out to be the bottom end, the next question is whether it is worth repairing at all. Our guide to engine failure and whether to repair or scrap works through that decision.
Grinding when you brake
Metal on metal. The friction material has gone and the backing plate is now cutting into the disc. Braking distance is affected and the discs are being destroyed while you drive.
This is the one where waiting until payday costs several times more than acting today. There is more in our guide to brake problems and when not to drive.
Squealing when you brake
Usually the wear indicator: a small metal tab designed to touch the disc and make exactly that noise when the pads get low. It is a warning, not an emergency. Book it in.
A squeal only on the first few stops of a damp morning is normal surface rust being cleaned off and means nothing.
Squealing that changes with engine speed
Not brakes. This is usually the auxiliary belt slipping — glazed, loose or perishing — or a failing pulley or tensioner. It often appears on start-up, in the wet, or with the air conditioning on.
Worth attending to sooner rather than later, because that belt commonly drives the alternator and the water pump. If it breaks you lose charging and, on many engines, coolant circulation, which becomes an overheating problem very quickly.
A rhythmic rumble or drone that rises with road speed
Classic wheel bearing. The giveaway is that it changes when you swing the car gently left and right at speed, because loading and unloading the bearing changes the noise. It can also be tyre wear, which sounds similar but does not usually change with steering input.
A wheel bearing does not fail politely once it is bad enough, so this is a soon job rather than a someday job.
Clicking or knocking on full lock
A regular clicking when turning tightly, most obvious in a car park, is very often a constant velocity joint on a front-wheel drive car. Usually caused by a split boot letting the grease out and the grit in. It will need doing, but it rarely leaves you stranded overnight.
Clunking over bumps
Suspension: worn drop links, bushes, top mounts. Annoying rather than immediately dangerous in most cases, but it is also what a car sounds like after a serious pothole impact, so if the noise started at a specific moment, say so.
Whining
A whine that rises and falls with engine speed can be the power steering pump, particularly if it is worse on full lock. A whine that tracks road speed and changes with gear selection points at the gearbox or differential. A high-pitched whine on an electric car is normal.
Hissing, ticking and roaring
Hissing after switching off can be normal cooling, but hissing with steam or a sweet smell means coolant is escaping and the engine is losing its ability to cool itself.
Ticking that rises with revs is often an exhaust manifold leak, which sounds alarming and generally is not.
A sudden roar that appeared all at once is usually the exhaust: a blown section or a dropped box. Not dangerous in the short term unless fumes are entering the cabin, which is the exception that means stop and ventilate.
The noises to stop for. Deep engine knocking, grinding brakes, anything accompanied by a red warning light, and any noise that arrived at the same moment as a change in how the car drives or steers. If you are not sure, describe it to us on the phone — the sound, when it happens, and what the car is doing — and we will tell you honestly whether it is a recovery job or something you can drive gently. DK’s Recovery covers Sheffield 24 hours a day on +44 7867 860723.
How to describe it so somebody can help
Say when it happens rather than what you think it is. Cold or warm. Accelerating, braking, coasting, turning. Constant or rhythmic. Whether it tracks road speed or engine speed. Whether it changed suddenly or crept up over weeks.
If you can safely record it on a phone from inside the car, do. A garage will get more from ten seconds of audio than from ten minutes of description, and so will we.
One noise arrives all at once rather than developing: a sudden, enormous roar from underneath with no other damage. That is usually theft rather than wear — see our guide to a stolen catalytic converter.
Frequently asked questions
What does a knocking noise from the engine mean?
A deep rhythmic knock that speeds up with the revs can indicate serious bottom-end wear, and driving on can end the engine. If it comes with an oil pressure warning light, stop. A lighter metallic rattle under acceleration is usually fuel or ignition related and less urgent.
Is grinding when braking dangerous?
Yes. Grinding means the friction material has gone and metal is cutting into the disc. Braking distances are affected and the discs are being destroyed as you drive. It is one of the few noises worth acting on the same day.
What is the difference between squealing and grinding brakes?
Squealing is usually the wear indicator, a small metal tab designed to touch the disc when the pads get low. It is a warning to book it in. Grinding means the pads have already gone and it is metal on metal.
How do I know if it is a wheel bearing?
A rhythmic rumble or drone that rises with road speed and changes when you swing the car gently left and right is the classic sign, because loading and unloading the bearing alters the noise. Tyre noise sounds similar but does not usually change with steering.
My car clicks when I turn tightly. What is that?
On a front-wheel drive car that is very often a constant velocity joint, usually because a split boot has let the grease out and grit in. It needs repairing but rarely leaves you stranded immediately.
Which noises mean I should stop driving?
Deep engine knocking, grinding brakes, anything alongside a red warning light, exhaust fumes entering the cabin, and any noise that appeared at the same moment as a change in how the car steers or brakes.
