Many noises thought to be coming from the rear axle may actually be originating from other sources such as the following:

Because noise is a major factor in determining or diagnosing a rear axle problem, a thorough and careful check should be made to determine the source of the noise before proceeding with rear axle repair.

Some road surfaces, such as brick or rough-surfaced concrete, cause noise which may be mistaken for tire or rear axle noise. Driving the vehicle on varying types of road surfaces such as smooth asphalt or dirt will help in determining whether road surface is the cause of the noise. Noise caused by road surface should be the same in drive or coast.

Tire noise can be easily mistaken for rear axle noise. Because noise can transmit or dissipate through the vehicle body, noise coming from the front tires can also be mistaken for rear axle noise. Tires showing uneven wear, or that are uninflated, are usually noisy and may produce vibrations which can appear at other places on the vehicle. This is particularly true with low tire pressure. For tire inspection procedures, Refer to Suspension in General Diagnosis.

The engine or transmission may sometimes produce noises which may be mistaken for rear axle noise. To determine which unit is actually causing the noise, run the following tests:

1. Record the speed, rpm and other conditions at which the noise is most pronounced.
2. Stop the vehicle in a quiet place to avoid interference with other extraneous noises.
3. With the transmission in neutral, slowly run the engine speed up and down to the engine speeds that correspond with the vehicle speed at which the noise was most pronounced. If the noise is produced with the vehicle at a standstill, the problem lies in the engine or transmission and not in the rear axle.

Most noises caused by the front or rear brakes are rotational and occur once every wheel revolution. During a road test, duplicate the conditions under which the noise is most pronounced and gently apply the brakes. If the noise immediately appears or disappears when lightly depressing the brake pedal, the noise may be caused by an out of round brake rotor or drum. For brake diagnosis and repair procedures, refer to Brakes.

Objectionable body boom noise or vibration usually occurs at 90 to 100 km/h (55 to 65 mph) and can be caused by the following:

If all indications point to a rear axle noise after making a comprehensive check of the vehicle, it is necessary to determine what component in the rear axle is at fault. True rear axle noise generally falls into one of the two following categories:

A rough rear axle shaft bearing produces a vibration or growl which continues with the vehicle coasting and the transmission in neutral. A brinelled rear axle shaft bearing causes a knock or click approximately every two revolutions of the rear wheel. Since the rear axle shaft bearing rollers do not travel at the same speed as the rear axle shaft, this condition can be diagnosed by supporting the rear axle with a hydraulic floor jack and spinning the rear wheels by hand and listening at the hub for evidence of a rough or brinelled rear axle shaft bearing.

Pinion Bearing Noise

Pinion bearing failures can be distinguished because pinion bearings rotate at a higher rate of speed than the differential side bearings or drive axles. Rough or excessively worn pinion bearings produce a continuous low pitched whirring or scraping noise on low speed acceleration from a stop.

Side Bearing Noise

Side bearings, when worn, produce a constant rough noise at a lower pitch than with pinion bearing noise. Side bearing noise may also fluctuate with drive axle shaft noise.

Side Gear and Differential Pinion Gear Noise

Side gears and differential pinion gears rarely cause noise. This is due mainly to their lack of movement during straight ahead driving. Noise produced by these gears will be most pronounced on turns.

Ring and Pinion Gear Noise

There are two types of gear noise. One type is produced by broken, bent, overheated or forcibly damaged gear teeth. This noise is usually quite audible through all speed ranges and can easily be diagnosed upon visual inspection. For example, hypoid gear tooth scoring generally results from the following:

The scoring will progressively lead to complete erosion of the gear teeth and, eventually, a fracture will result if the initial scoring condition is not corrected. Another common cause of hypoid gear tooth fracturing is extended overloading or shock loading--causing the differential pinion gears and shaft to seize. This can result from lubrication breakdown caused by excessive wheel spin. The second type of gear noise pertains to the contact pattern of the ring and pinion gear teeth. This form of abnormal gear noise can be recognized because it produces a cycling pitch (whine). This whine will be very pronounced at the speed range in which it occurs. Because the front differential is driven (powered) in four-wheel drive only, this noise can only be duplicated in four-wheel drive. Gear noises in the front differential can occur under one or all of the following conditions:

Gear noises tend to peak at a narrow speed range or ranges and tend to remain constant in pitch. Bearing noises will vary in pitch with vehicle speeds. For further diagnosis, refer to the noise diagnosis tables.