Humming noise from under the car: wheel bearing, driveshaft or tires?

Picture the following scenario, which illustrates how this complaint typically develops. At first you can barely hear it. At 40 km/h there is a faint hum from under the floor, a bit like tires on rough asphalt. A few weeks later, on the toll road, the hum has grown into a roar that drowns out the radio. Say the owner replaces the tires, but the noise stays; one workshop suspects a bearing, another the driveshaft.

A hum from under the car really is hard to trace from the driver's seat, because the wheel bearings, the tires and the driveshaft all turn in step with road speed. The good news: a few simple road tests can narrow things down before the car ever goes up on a lift.

The wheel bearing carries the wheel on its axle. A worn bearing makes a hum or growl that gets louder with speed and sometimes turns into a grinding noise from the wheel area. It usually seems to come from one side.

Tires with uneven wear also make a hum that rises with speed. The usual causes are a scalloped tread (cupping), tread blocks worn at an angle (feathering), a flat spot, or low air pressure. Cupping is often the result of worn-out shock absorbers or struts.

The driveshaft carries drive from the transmission to the differential in rear-wheel-drive cars and many 4x4s. It has universal joints (U-joints, also called cross joints), and a two-piece shaft also has a center bearing. A worn U-joint is more likely to clunk and squeak than hum. A worn center bearing, though, can make a hum, a low roar or a growl that changes with speed.

On a quiet, straight road, at a steady speed where the hum is clearly audible, ease the steering wheel a little to the left, then to the right. You only want to shift the car's weight to one side, not to make a real turn.

• If the noise gets louder in one direction and fades in the other, it is almost certainly a wheel bearing. A worn bearing protests when it takes more load.
• If the noise does not change at all, unevenly worn tires are the more likely cause.

This test tells you what kind of problem you have. Which side it is on should be confirmed at a workshop with the wheels off the ground, not just by ear.

Get up to the speed where the noise is clear, then lift your foot off the gas pedal and let the car coast.

• If the hum stays the same, the noise follows wheel speed: a bearing or the tires.
• If the hum vanishes the moment you coast, the noise follows drive load: the differential or the driveline.

A noise that is louder when accelerating than when cruising steadily also points to the differential or the driveshaft. In an automatic, you can briefly shift to N at that speed. If the noise does not change as the engine revs drop, the source is not the engine or transmission. Only do this on a safe, empty road.

Tire hum follows the road surface: louder on rough asphalt, quieter on smooth asphalt, and indifferent to where the steering wheel points. Bearing hum sounds much the same on any surface. If the noise barely changes between two different surfaces, a wheel bearing, dragging brakes, the driveline or exhaust resonance is more likely than the tires.

One more clue: if you rotate the front tires to the rear and the hum moves with them, the tires are the source. Bearing noise stays at the same corner of the car, wherever the tires are fitted.

• A clunk when you shift from D to R or back, or when you first press the gas from a stop and when you lift off.
• A squeak at low speed that rises in pitch as you speed up, the sign of a dry U-joint short of grease.
• Vibration you feel through the floor or center console, often at moderate speeds, or when pulling away from a stop if the center bearing is damaged.

U-joints usually wear out from poor lubrication, normal wear sped up by heavy loads or rough terrain, loose bolts, a changed shaft angle after suspension or tire-size modifications, and a dented shaft. On the center bearing, the rubber mount fails more often than the bearing itself: the rubber hardens, cracks and then tears, letting the shaft swing and strike its housing.

1. Run your hand over the tread while the car is parked: feel for waves, sawtooth edges on the tread blocks, or flat spots.
2. Check the air pressure in all four tires.
3. Notice the steering wheel: vibration through the wheel at a particular speed is more often down to wheel or tire balance.
4. Watch for other bearing symptoms: the steering feels loose or vague, the car pulls to one side, or there is play when the wheel is rocked.

Further checks need the car raised. Each wheel is spun and its noise and drag are compared left to right, and the driveshaft is rocked near the U-joints to feel for play or a clunk.

Not every hum from under the car comes from the three main suspects. These are often mistaken for them:

• Differential. A failing differential often whines, roars or growls, with a pitch that changes with speed. The clue: it is louder under acceleration and disappears when you coast.
• Dragging brakes. Pads that do not fully release from the disc can cause a hum or rubbing noise, and usually make one wheel much hotter than the others.
• Exhaust resonance. The exhaust can resonate and sound like a hum, so it is worth checking once the bearings and tires have been cleared.
• Wheel balance. Vibration in one speed range, especially through the steering wheel, more often comes from an unbalanced wheel than from a bearing.

A workshop will usually repeat your road tests, then put the car on a lift. Each wheel is spun by hand to hear and feel any roughness in the bearing, and compared with the wheel on the other side. The wheel is also rocked to check for play. The tread is checked for wear patterns. If it is cupped, the shock absorbers and suspension parts such as ball joints and tie rods are checked too, because tire wear is often a symptom of a suspension problem.

On rear-wheel-drive and 4x4 cars, the driveshaft is rocked near each U-joint to check for looseness, and the bearing caps are checked for rust trails that show the grease has run out. On a two-piece shaft, the center bearing is checked along with every U-joint so the wrong part is not replaced.

A wheel bearing that is left alone keeps getting worse and can make the car unsafe to control; a worn bearing also speeds up tire wear, and the reverse is true too. A U-joint that has play or clunks needs replacing soon. Stop using the car if you find obvious looseness in the shaft, a bearing cap that moves, or a loud clunk together with vibration. If the hum is still faint, book a check soon and avoid high speeds until the source is confirmed.