What’s That Humming Noise

It is Something I Should be Concerned About?

If you are driving along and you hear a humming noise above the sound of the engine, you may well have a wheel bearing that is on its way, and that’s something which needs checking out.

The way to tell is this. Whilst driving along, if you put your foot on the clutch, drop the revs and coast for a few seconds, does the noise stop? If not, there is good chance this is a wheel bearing problem.

Another test is to consider if the noise increases its intensity when you are cornering in one direction by comparison to the opposite direction. If it does, this again strongly suggests you have a worn wheel bearing that needs replacement.

More often than not it will be a front wheel bearing that is the problem, and it will be an inner bearing (each wheel has two, but we will look at that in a moment).

To identify which one is at fault, if the humming or droning noise gets louder as you drive a left hand bend, it is most likely be the front off–side (driver’s side) inner bearing that is at fault. It therefore follows that if you are cornering to the right, it is the nearside front inner bearing.

The wheel bearings on a car are what are called Tapered Roller Bearings, and when you look at the diagram you will see where the name comes from. The whole thing is tapered at an angle, and fits into the ring you can see accompanying it, which obviously has a tapered recess into which it fits. The ring is called a Bearing Race, and it is the internal surface around which the rollers travel to provide the smooth rotation of the wheel.

Take a look at the picture of the stub axle. The shaft that is sticking out is what your wheel hub is located onto. You will note the shaft is thicker at the inboard end and the thinner outer end has a screw thread.

The two bearings will differ in size, and the larger one fits on the fatter part of the stub axle whilst the smaller one fits over the thinner end, before the whole lot is held in place by a large nut that screws onto the threaded end.

What Happens If I Ignore The fault?

Not a good idea. When the bearings begin to deteriorate, all those rollers you see in the picture begin to loosen, go out of round through skidding rather than rolling, and can get quite flat sides to them.

Mishapen rollers are what make the droning noise as they are not running smoothly. The more rough they get the more heat is built up, and with heat the grease, which is used to lubricate the bearing, begins to melt.

As the grease turns to liquid it eill run out of the bearing, the bearings themselves get hotter and wear more quickly, thus melting the grease more quickly.

If you leave a worn bearing long enough like this it will break up, so that the rollers break loose from their cage and can even twist sideways within the bearing housing. With the immense heat this causes your stub axle may not be able to cope and actually sheer off from its mounting — taking your wheel with it.

Worn or loose bearings is an MOT failure fault

There is one thing you can do that will wreck wheel bearings very quickly, and that is by driving with your handbrake on. Daft as you might think that sounds, you thinking how could anyone do that, you don’t necessarily have to have the handbrake lever in the applied position to do it.

The handbrake works by cables, just like thicker and more heavy duty versions of bicycle brake cables, and there are two – one to each back wheel. As these are suspended under the floor of the car they are exposed to all the muck and water that is flung around when you drive on a wet road. This can cause corrosion to the cables and that can actually make then jam. If this happens, whilst you let the handbrake off in the car, at the wheels it can still be applied.

You won’t realise, of course, and as you drive, with your back brakes still applied, they are going to get very hot. This situation heats up the wheel bearings, the grease melts and runs out, the bearings run dry and then they disintegrate. That is when your wheel is likely to come off.

Parking up over night in freezing temperatures, and after driving in the rain, can cause the same problem as the water that penetrates the handbrake cables may freeze and not allow the rear brakes to release.

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This page was last updated
Wednesday, 02-Feb-2011

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