Humming, clicking or grinding: bearing, CV joint or tyre?

Start by ruling out the free one

Before anything is condemned, rotate the tyres front to back and drive the same road again.

Cupped, feathered or unevenly worn tyres produce a hum that is almost indistinguishable from a bearing, and it is far more common than people expect — particularly on a car that has been out of alignment or has had worn suspension bushes. If the noise moves to the other end of the car, or changes character, it was the tyres.

This test costs nothing and it saves a considerable number of unnecessary hub assemblies.

The four noises

A hum or growl that rises and falls with road speed. Not with engine rpm — this is the distinction that matters. Change gear at a constant speed and see whether the noise changes. If it tracks the speedometer and ignores the tachometer, it is rotational and it is at a wheel, a driveshaft or the differential.

A rhythmic click or knock on full lock at low speed. Reverse around a car park on full lock with the window down and it is unmistakable. This is an outer CV joint, and it is the single most recognisable noise in the whole category.

A shudder or vibration under acceleration, worst pulling away from a stop, uphill, or with the wheels turned. This is an inner CV joint — the plunge joint — where the tripod has worn its tracks.

A whine that changes when you lift off the throttle at a steady speed. Load-sensitive rather than speed-sensitive. That is a differential or transfer case, and it is a different job entirely.

Confirming a bearing

The sway test is the one worth learning. On a quiet road at around 40 mph, gently weave the car left and right — a lane change without the lane change, not a swerve.

Weight transfers to the outside of each movement. A worn bearing gets louder when it is loaded. So a noisy left bearing usually gets louder when you steer right, and quieter when you steer left.

With the wheel off the ground:

• Rock the wheel at 12 and 6 o'clock. Play here is either a bearing or a ball joint — have someone press the brake pedal to take the bearing out of the equation, and if the play disappears it was the bearing.
• Spin it by hand and listen with a stethoscope or a long screwdriver against the knuckle. A dry, gritty rumble is unambiguous.
• Look for a grease film thrown out around the hub seal.

Confirming a CV joint

• Turn the wheel to full lock and look at both boots, inner and outer. A split, a tear, or a spray of grease across the inside of the wheel arch is the diagnosis.
• Grip the shaft near the outer joint and try to rotate it against the hub. Any perceptible rotational play in the joint is wear.
• With the vehicle safely supported, listen while the wheel is driven at a low speed and turned — the click follows the rotation.

A boot that is torn and a joint that is quiet is a race against time. A joint that clicks has already ingested grit and lost grease, and the axle is the part.

What is actually being replaced

Hub assemblies. Most modern cars use a bolt-on hub unit — bearing, hub, flange and often the ABS encoder in a single sealed assembly. It is not serviceable and not repackable. This is a good thing: it is a bolt-off, bolt-on job with a torque spec, rather than a press and a set of races.

There is a specific trap on these, which is the magnetic encoder built into the bearing seal. It is covered in its own guide, and getting it wrong gives you an ABS light with no visible damage.

CV axles. Sold as a complete shaft with both joints and both boots. Replacing the whole shaft is standard practice because the labour to remove it is the same either way, and a shaft with one worn joint usually has a second joint the same age.

Fit them in pairs where the vehicle's use justifies it. One new axle and one original is not a fault, but the original one is the same age as the one that just failed.

Two details that cause comebacks

The axle nut. It is a critical torque and on most vehicles it is single-use — staked, or with a deforming thread lock. Under-torque leaves the bearing with play and it will be back within months. Over-torque destroys the preload. Use a torque wrench and a new nut.

Never hammer through a bearing. Driving a stuck shaft out by hitting it transmits the impact through the bearing races and brinells them. The bearing will be quiet on the day and noisy in three months, and it will look like the new part failed.