Noises are hard to diagnose from a description because everybody describes them differently. One person’s rumble is another person’s drone. What is much more reliable than the sound itself is the behaviour of the sound — what makes it louder, what makes it go away, whether it follows road speed or engine speed.

That is the useful question, and it splits noises into four groups. Working out which group yours is in does most of the diagnostic work.

Group One: It Changes With Road Speed

The noise rises and falls with how fast the car is going, regardless of what gear you are in or what the engine is doing. Coast in neutral at the same speed and it is still there. That points at something rotating with the wheels — bearings, tyres, driveshafts, brakes.

Wheel bearing. The classic is a hum or drone that comes in around 30 to 40mph and gets louder with speed, often described as sounding like driving on a rough surface when the road is smooth. The giveaway is what happens on a curve: load a bearing and it gets worse, unload it and it improves. On a long left-hand bend, weight transfers to the right-hand wheels, so a right-hand bearing gets louder going left and quieter going right. A gentle sweeping curve on a dual carriageway is the ideal place to test it. A badly worn bearing eventually growls at low speed too, and you may feel a vibration through the floor or the seat.

Tyres. Tyre noise also follows road speed and can sound very like a bearing, which catches people out. Unevenly worn tyres, particularly with feathered or scalloped wear, produce a drone that changes with road surface — it disappears on fresh tarmac and comes back on a coarse surface, whereas a bearing does not care what the road is made of. Swapping wheels front to rear is a crude but effective way to find out.

Brakes. A cyclical scrape or light rubbing that slows with the car, sometimes with a squeal, usually means a pad worn to its backing, a corroded disc, or a pad binding in its carrier. A rhythmic noise that comes round once per wheel revolution is a disc problem rather than a bearing.

Group Two: It Changes With Engine Speed

The noise follows the rev counter. Rev it in neutral and it is there; coast at speed with the engine idling and it is not. That means something driven by the engine — the belt, an ancillary, the exhaust, or the engine itself.

Belt squeal. A sharp squeal on a cold start that eases as things warm through is nearly always the auxiliary belt — glazed, perished, or a tensioner that has lost its grip. It is often worse in damp weather and when the heater blower or air conditioning loads the belt. Cheap to sort, and worth sorting: the belt drives the alternator and the water pump, and a snapped one leaves you with no charging and no cooling.

Exhaust blow. A puffing, rasping or blowing noise that rises with revs and is loudest under load, often with a deeper drone in the cabin. A blow at a joint or a failed gasket usually sounds tinny and tapping at idle; a hole further back sounds boomy. Rusted-through sections are common on cars that see salted roads, and it is an MOT failure as well as a noise.

A rhythmic tapping that follows revs can be a number of things — valve clearances, a noisy injector, a heat shield vibrating at a particular engine speed. Heat shields are worth mentioning because they cause a lot of alarming rattles for nothing.

A deep knock or rumble from low in the engine that follows revs and gets worse under load is the one to stop for. That is bottom-end territory and it does not improve by being driven.

Group Three: It Changes With Steering

The noise appears when you turn, and the amount of lock matters.

CV joint. A rhythmic clicking or knocking on full or near-full lock, most obvious pulling out of a junction or manoeuvring in a car park, and usually only in one direction. It is a rapid tick-tick-tick that speeds up as the wheel turns faster. Have a look at the rubber CV boot behind the wheel while you are there — a split boot slinging grease around the inside of the arch is what killed the joint, and catching a split boot early saves the joint entirely.

Power steering. A groan or whine when turning, worst at low speed and full lock. On hydraulic systems that is often low fluid or a tired pump; on electric systems it is more likely the motor or its mounting.

Top mount or strut bearing. A creak, clonk or graunching noise as you turn the wheel while stationary or nearly so, felt through the steering wheel as much as heard.

Group Four: It Happens Over Bumps

The noise is tied to the road surface, not to speed or revs. Smooth road, silence. Rough lane, a clatter. This is suspension.

Anti-roll bar drop links are the single most common cause, and they are worth knowing about because the noise they make is out of all proportion to the part. A worn drop link produces a light knocking or rattling over small, sharp bumps — cats’ eyes, drain covers, patched tarmac — and it is often more noticeable at low speed than at high. They are inexpensive and quick to change.

A heavier clonk over larger bumps points more at worn bushes, a top mount, or a ball joint. A creaking as the suspension moves, particularly reversing off a driveway or over a speed hump, is usually a dry or perished bush.

A rattle from underneath that comes and goes is very often an exhaust mounting rubber that has perished, letting the system knock against the body — another cheap fix that sounds expensive.

What Is Urgent And What Can Wait

Not every noise is a reason to stop driving, and it helps to know which is which.

Stop now and do not drive it:

• A deep knocking from the engine that follows revs, especially with low oil pressure or a warning light.
• Any grinding or metallic scraping when braking, or a noise accompanied by a soft or long-travelling brake pedal.
• A loud clonk with vague or wandering steering, or a noise that arrived along with a change in how the car steers.
• A screeching belt that then goes quiet, with the battery light on — the belt has gone and you have no charging or water pump.
• Anything accompanied by a burning smell, smoke, or a temperature gauge climbing.

Book it in soon, keep the mileage down: a humming wheel bearing, a clicking CV joint, an exhaust blow, brakes that squeal without grinding. These are deteriorating and will get more expensive — a bearing that is allowed to go long enough can damage the hub, a CV joint can fail completely and leave you without drive — but they are not usually an immediate danger over a few days.

Mention it at the next service: a knocking drop link, a creaking bush, a rattling heat shield or exhaust mounting, a belt squeal that only appears on the first cold start. Annoying rather than dangerous, and cheap to sort when the car is already up.

Describing It Well Saves You Money

Diagnostic time is time, and a good description shortens it. When you ring, the things worth having ready are:

1. What makes it change — road speed, engine speed, steering, bumps, braking. This is the most valuable single piece of information.
2. Where it seems to come from — front or back, left or right, and whether you feel it as well as hear it.
3. When it started, and whether anything happened at the time: a pothole, a kerb, a cold snap, a recent repair.
4. Whether it is getting worse, and how quickly.
5. What it does not do — “it never does it in the dry”, “it goes away above 50”, “only when the car is cold”. Negatives narrow things down as effectively as positives.

If you can, record it on your phone from inside the car while a passenger drives, and say out loud what you are doing as it happens. A ten-second clip with “that’s at about 40, coming into a left-hander” is worth more than a paragraph of description.

Being able to reproduce the noise on demand is the other thing that helps enormously. A noise that only happens once a fortnight is genuinely hard to find; a noise that appears every time you take the same corner is straightforward. We can come out and drive it with you on the road where it happens, which is often how a diagnosis and repair gets sorted in one visit rather than two. Anything that needs the car properly raised gets booked in with us.