Any gear-driven unit produces a certain amount of noise. This includes an automotive drive axle where engine torque multiplication occurs at a 90 degree turn in the driveline. Perform an interpretation for each vehicle in order to determine whether the noise is normal or if a problem exists. Expect a certain amount of noise that cannot be eliminated by conventional repairs or adjustments.

Normal rear axle noise is a slight noise that is only heard at a certain speed or under unusual or remote conditions. Noise tends to reach a peak at speeds from 64-97 km/H (40-60 mph) depending on the road and load conditions or on the gear ratio and the tire size. This type of noise is not indicative of trouble in the rear axle.

The following vehicle noises are often mistaken for rear axle noise:

The following practices will aid in locating the source of rear axle noises:

Isolate the noise to a specific driveline component. Do not make a random guess. Random guessing may be a costly waste of time.

If you suspect that a rear axle is noisy, perform a thorough test in order to determine whether the noise originates from one of the following locations:

Rear axle parts adjustment or replacement will not correct noise that originates in other locations.

Driving on the following road surfaces causes noise which may be mistaken for rear axle noise:

Driving on one of the following different types of road surface will quickly show whether the road surface is the cause of the noise:

Road noise is usually the same on drive or on coast.

Tire noise may easily be mistaken for rear axle noise, even though the noisy tires may be located on the front wheels. Tires that exhibit the following conditions are usually noisy and may produce vibrations which seem to originate elsewhere in the vehicle:

Tire noise changes according to different road surfaces, but rear axle noise does not. Temporarily inflating the tires to the maximum recommended pressure, for test purposes only, will materially alter the tire noise. Temporarily inflating the tires will not affect the noise caused by the rear axle. Rear axle noise usually stops when coasting at speeds under 48 km/h (30 mph). Tire noise may continue at a lower tone as you reduce the vehicle speed. Rear axle noise usually changes when comparing the pull and the coast conditions. The tire noise remains about the same.

Sometimes a noise which seems to originate in the rear axle is actually caused by the engine or transmission. In order to determine which unit is causing the noise, complete the following steps:

1. Observe the approximate vehicle speeds and conditions under which the noise is most pronounced.
2. Stop the vehicle in a quiet place in order to avoid interfering noises.
3. Place the vehicle in Neutral.
4. Run the engine slowly up and down through the engine speeds that correspond to the vehicle speed at which the noise is most pronounced.

If a similar noise is produced with the vehicle standing, the noise is caused by the engine or the transmission and not the rear axle.

Do not confuse loose or rough front wheel bearing noise with rear axle noise. Front wheel drive noise does not change when comparing pull and coast conditions. Light application of the brake, while holding the vehicle speed steady, will often cause the front vehicle noise to diminish. This light application of the brake takes some weight off of the bearing. Inspect the front wheel bearings for noise. Complete the following steps:

1. Raise and spin the front wheels.
2. Shake the front wheels in order to determine if the front wheel bearings are excessively loose.

An unbalanced propeller shaft may cause an objectionable body boom noise or vibration at 89-105 km/h (55-65 mph). Excessive looseness at the propeller shaft spline may contribute to the imbalance. The following items also contribute to the noise problem:

After completing a comprehensive check of the vehicle, if all indications point to the rear axle, further diagnostic steps are necessary in order to determine the rear axle components are at fault. True rear axle noise generally falls into the following two categories:

If a careful test of the vehicle shows that the noise is not caused by external items, the rear axle is the cause of the noise. Test the rear axle on a smooth level road in order to avoid road noise. Do not test the rear axle for noise by running the vehicle with the rear wheels raised.

Noise in the rear axle may be caused by one of the following conditions:

A rough rear wheel bearing produces a vibration or a growl which continues while the vehicle is coasting and the transmission is in Neutral. A brinelled rear wheel bearing causes a knock or click approximately every two revolutions of the rear wheel. This occurs because the bearing rollers do not travel at the same speed as the rear axle and the rear wheel. Raise the rear wheels and spin the rear wheels by hand. Listen at the rear wheel hubs for evidence of a rough or brinelled rear wheel bearing.

Differential side gear movement is relatively slight on straight ahead driving and may seldom cause noise. Noise from the gears is most pronounced during turns.

In order to determine pinion bearing failures compare the bearings rotation at higher speeds to the differential side bearings and axle shaft bearings.

Rough or brinelled pinion bearings produce a continuous low-pitched whirring or scraping noise starting at a low speed.

Side bearings produce a constant rough noise pitched lower than pinion bearing noise. Side bearing noise may also fluctuate in the previous rear wheel bearing test.

There are two basic types of gear noise. Broken, bent or damaged gear teeth produce the first type of noise. The gear noise is usually audible over the entire speed range and does not present a problem in diagnosis. Hypoid gear tooth scoring generally results from the following conditions:

The scoring will progressively lead to the complete erosion of the gear tooth, or the gear tooth pitting. Fracture of the gear tooth will eventually occur if the scoring condition is not corrected.

Other causes of the hypoid tooth fracture include the following items:

Differential pinion gear and side gears normally do not fail. Common causes of differential malfunctions include the following conditions:

The second type of gear noise pertains to the mesh pattern of the gear teeth. This form of abnormal gear noise produces a cycling pitch (whine). The gear noise will be very pronounced at the speed range at which the noise occurs. This pitch (whine) appears under the following conditions:

Bearing noise will vary in pitch with the vehicle speeds.