Determining Type of Noise - Rear Drive Axle - Conventional

REAR AXLE NOISE
In axle work, one of the most difficult areas to diagnose is noise. Locating a broken axle shaft or broken differential gear presents little or no problem, 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 to determine whether the noise is normal or if a problem actually exists. A normal amount of noise must be expected and cannot be eliminated by conventional repairs or adjustment.
Acceptable axle noise can be described as a slight noise heard only at a certain speed or under certain 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.

EXTERNAL NOISE
When a rear axle is suspected of being noisy, make a thorough test to determine whether the noise originates in the tires, road surface, front wheel bearings, engine, transmission or rear axle assembly. Noise which originates in other places cannot be corrected by adjustment or replacement of parts in the rear axle assembly.

ROAD NOISE
Some road surfaces, such as brick or rough-surfaced concrete, can cause 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 conditions as in coast conditions.

TIRE NOISE
Tire noise may easily be mistaken for rear axle noise, even though noisy tires may be located on the front wheels. Tires worn unevenly wear in a sawtooth fashion. Tires worn unevenly 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 TEST
Tire noise changes with different road surfaces, but rear axle noise does not. Temporarily inflating all tires to 345 kPa (50 psi) pressure, for test purposes only, will 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 48 km/h (30 mph). However, tire noise continues but with 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, but tire noise remains about the same.

ENGINE AND TRANSMISSION NOISES
Sometimes a noise which seems to originate in the rear axle is actually caused by the engine or transmission. To determine which unit is actually causing the noise, observe 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, it is caused by the engine or transmission and not the rear axle.

FRONT WHEEL BEARING NOISE
Loose or rough front wheel bearings will cause noise which 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 to determine if the bearings are excessively loose.

BODY BOOM NOISE OR VIBRATION
Objectionable body boom noise or vibration at90 - 100 km/h (55 - 65 mph) 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 include:
^ Undercoating or mud on the shaft, causing imbalance
^ Missing shaft or companion flange balance weights
^ Shaft damage, such as bends, dents or nicks
^ Rough tires (Switch tires from a known good vehicle in order to determine a tire fault.)
If after making a comprehensive check of the vehicle, all indications point to the rear axle, further diagnostic steps are necessary to determine the axle components at fault. True rear axle noise generally falls into two categories: gear noise and bearing noise.

REAR AXLE NOISE
If a careful test of the vehicle shows that the noise is not caused by external items, it is then reasonable to 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 the two universal joints or worn differential side gears and pinions. Noise may be caused by a mismatched, improperly adjusted, or scored ring and pinion gear set.
Noise in the rear axle assembly may be caused by a faulty propeller shaft, faulty wheel bearings, fault differential or pinion shaft bearings, misalignment between the two universal joints or worn differential side gears and pinions. Noise may be caused by a mismatched, improperly adjusted, or scored ring and pinion gear set.

REAR WHEEL BEARING NOISE
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 GEAR AND PINION NOISE
Differential side gears and pinions seldom cause noise since their 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 because the bearings rotate at higher speeds than the differential side bearings and axle shaft bearings do. 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 is. Side bearing noise may also fluctuate in the previous rear wheel bearing test.

GEAR NOISE





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.





Hypoid tooth fracture is caused by extended overloading of the gear set, shock loading, and gear tooth scoring (2), which produce fatigue fracture, resulting in gear set failure.
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 then eventual fracture, if the initial scoring condition is not corrected.
Differential pinion and side gears rarely give trouble. Common causes of differential malfunction are shock loading, extended overloading and seizure of the differential pinions to the cross shaft resulting from excessive wheel spin and consequent lubrication breakdown.
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 a particular speed range. The gear noise can occur in either drive, float or coast conditions. Drive is acceleration or heavy pullout. Coast is when no acceleration takes place and the vehicle is in gear. Float happens when just enough acceleration occurs 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. Refer to Wheel Bearings Diagnosis.

DRIVELINE NOISE
Driveline noises may baffle even the best technician. Vehicle noises coming from the tires, transmission, propeller shaft, universal joints and front or rear wheel bearings, are often mistaken for axle noise.
Such practices as raising the tire pressure 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 (in drive, float or coast conditions) will aid locating the source of alleged axle noises. Every effort should be made to isolate the noise to a specific driveline component instead of making a random guess that could be a costly waste of time.