The proper diagnosis is an important part of rear axle repair. In axle work, one of the most difficult conditions to diagnose is noise. Locating a broken axle shaft or broken differential gear presents little or no problems, but locating and isolating axle noise can be an entirely different matter.

Any gear driven unit, especially an automotive drive axle where the engine torque multiplication occurs at a 90 degree turn in the driveline, produces a certain amount of noise. Therefore, an interpretation must be made for each vehicle in order to determine where the noise is normal or if a problem actually exists. A certain amount of noise must be expected and cannot be eliminated by conventional repairs or adjustment.

Normal axle noise can be described as a slight noise heard only at a certain speed or under unusual or remote conditions. For example, the noise tends to reach a peak at speeds from 60 to 100 km/h (40 to 60 mph) depending on road and load conditions, or on gear ratio and tire size. This slight noise is in no way indicative of trouble in the axle assembly.

Driveline noises may confuse even the best technician. Vehicle noises coming from tires, transmission, propeller shaft, universal joints, and front or rear wheel bearings are often mistaken for axle noise. Such practices as raising tire pressure in order to eliminate tire noise (although this will not silence tread noise of mud and snow tires), or listening for the noise at varying speeds and road surfaces (drive, float, and coast conditions), will aid in locating the source of alleged axle noises. Isolate the noise to a specific driveline component.

Noise which seems to be coming from the rear axle may actually be produced somewhere else. Determine whether the noise might originate in the tires, the road surface, the front wheel bearings, the engine, or the transmission.

Driving on certain road surfaces, such as brick or rough-surfaced concrete, causes noise which may be mistaken for tire or rear axle noise. Driving on a different type of road, such as smooth asphalt or dirt, will quickly show whether the road surface is the cause of noise. Road noise usually is the same in drive (D) as in neutral (N).

Tire noise can easily be mistaken for rear axle noise, even though noisy tires may be located on the front wheels. Tires worn unevenly, or having surfaces on non-skid divisions worn in sawtooth fashion, are usually noisy and may produce vibrations which seem to originate elsewhere in the vehicle. This is particularly true with low tire pressure.

Tire noise changes with different road surfaces, but rear axle noise does not. Temporarily inflating 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 rear axle. Rear axle noise usually stops when coasting at speeds under 30 mph; however, tire noise continues with a lower tone as the vehicle speed is reduced. Rear axle noise usually changes when the tire pulls in a forward direction or when the vehicle coasts down the road and tire noise remains about the same.

Sometimes a noise which seems to originate in the rear axle is actually caused by the engine or the transmission. In order to determine which unit is actually causing the noise, observe the approximate vehicle speeds and conditions under which the noise is the most pronounced; then stop the vehicle in a quiet place to avoid interfering noises. With the transmission in neutral, run the engine slowly up and down through the engine speeds corresponding to the vehicle speed at which the noise was most pronounced. If a similar noise is produced with the vehicle standing, the noise is caused by the engine or transmission and not the rear axle.

Loose or rough front wheel bearings will cause noise that may be confused with rear axle noise; however, front wheel bearing 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 wheel bearing noise to diminish, since this takes some weight off the bearing. Front wheel bearings may be easily checked for noise by jacking up the wheels and spinning them, and also by shaking the wheels in order to determine if the bearings are excessively loose.

Objectionable body boom noise or vibration at 55-65 mph (90-100 km/h) can be caused by an unbalanced propeller shaft. Excessive looseness at the spline can contribute to this unbalance.

Other items that may also contribute to the noise problem are the following:

If, after making a comprehensive check of the vehicle, all indications point to the rear axle, further diagnostic steps are necessary in order to determine the axle components at fault. True rear axle noise generally falls into two categories: gear noise and bearing noise.

If a careful test of the vehicle shows that the noise is not caused by external items, assume that noise is caused by the rear axle assembly. 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 with the rear wheels jacked up.

Noise in the rear axle assembly may be caused by a faulty propeller shaft, faulty wheel bearings, faulty differential or pinion shaft bearings, misalignment between two U-joints, or worn differential side gears and pinions. Noise may be caused by a mismatched, improperly adjusted, or scored ring and pinion gear set.

A rough rear wheel produces a vibration or growl which continues with the vehicle coasting and the transmission in neutral. A brinelled rear wheel bearing causes a knock or click, approximately every two revolutions of the rear wheel, since the bearing rollers do not travel at the same speed as the rear axle and wheel. With the rear wheels jacked up, spin the rear wheels by hand while listening at the hubs for evidence of a rough or brinelled wheel bearing.

Differential side gears and pinions seldom cause noise since the movement is relatively slight on straight ahead driving. Noise produced by these gears will be most pronounced on turns.

Pinion bearing failures can be determined by how the bearings rotate at higher speeds as opposed to 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.

The first type is produced by broken, bent, or forcibly damaged gear teeth (1) and is usually quite audible over the entire speed range and presents no particular problem in diagnosis. For example, hypoid gear tooth scoring generally results from the following: insufficient lubricant, improper break-in, incorrect lubricant, insufficient gear backlash, improper ring and pinion gear alignment, or loss of drive pinion nut torque. The scoring will progressively lead to complete erosion of the gear tooth, or gear tooth pitting and eventual fracture if the initial scoring condition is not corrected.

Other causes of hypoid tooth fracture are extended overloading of the gear set which will produce fatigue fracture, or shock loading which will result in sudden malfunction.

Differential pinion and side gears rarely give trouble. Common causes of differential malfunction are shock loading, and seizure of the differential pinions to the cross shaft resulting from excessive wheel spin and consequent lubrication breakdown (2).

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 in the speed range at which it occurs, appearing under either drive, float or coast conditions. Drive is acceleration or heavy pullout. Coast is allowing the vehicle to roll down the road without accelerating and float is lightly stepping on the accelerator pedal in order to keep the vehicle from driving the engine; the vehicle slows down gradually but the engine still pulls slightly. Gear noise tends to peak in a narrow speed range or ranges and will tend to remain constant in pitch. Bearing noise will vary in pitch with vehicle speeds.