Some road surfaces cause noise that may be mistaken for tire or axle noise. Road noise is usually the same while driving or coasting. Driving on a smooth surface helps determine the cause of the noise.

Perform the following maintenance/diagnostic checks:

Tire noise continues with lower tone as the vehicle slows down. Tire noise may easily be mistaken for axle noise. Tire noise changes with varying road surfaces. Axle noise does not change, although, it usually stops when coasting at speeds under 48 km/h (30 mph). Temporarily inflating all the tires to 345 kPa (50 psi) pressure for test purposes only will materially alter noise caused by tires, but will not affect noise caused by the axle.

Perform the following maintenance/diagnostic checks:

In order to determine which unit is at fault, take note of approximate vehicle speeds and conditions under which the noise is most pronounced. Stop the vehicle in a quiet place to stop interfering noises. With the transmission in neutral, run the engine slowly throughout the range corresponding to the vehicle speed at which the noise was most pronounced. If a similar noise is produced, the noise is caused by the engine or transmission and not the axle.

Perform the following diagnostic checks:

1. Drive slightly above the speed where the noise occurs. Place the transmission in neutral.
2. Let the engine speed drop to idle.
3. Stop the vehicle.
4. Run the engine at various speeds.

Raise the vehicle on a twin post hoist. Spin the wheels by hand. Listen for a knock or a click about every two revolutions of the wheel. The bearing rollers do not travel at the same speed as the axle and the wheel.

Perform the following diagnostic checks:

1. Drive the vehicle at low speed on a smooth road.
2. Turn to the left and turn to the right. The noise should change with the cornering loads.
3. Jack up the wheels in order to verify roughness at the wheels.

There are two basic types of gear noise. The first type is produced by broken, bent, or forcibly damaged gear teeth which is usually quite audible over the entire speed range, and presents no difficulty in diagnosis. Hypoid gear tooth scoring is one example that generally results from the following conditions:

This scoring will progressively lead to complete erosion of the gear teeth or gear teeth pitting and then eventual fracture if the initial scoring is not corrected. Another cause of hypoid gear fracture is extended overloading of the gear set. This will produce fatigue fracture, or shock loading, which will result in sudden failure.

The second type of gear noise pertains to the mesh pattern of the gear teeth. This form of abnormal gear noise can be recognized because it produces a cycling pitch (whine) and will be very pronounced. It tends to peak in a narrow speed range, appearing under drive, float, or coast conditions and will remain constant in pitch. Bearing noise will vary in pitch with vehicle speeds.

Perform the following diagnostic check: