Noise Diagnosis

GENERAL INFORMATION-NOISE DIAGNOSIS
The most important part of rear axle service is correctly identifying the cause of failures and noise complaints. The cause of most rear axle failures is relatively easy to identify. However the cause of rear axle noise is much more difficult to identify.

If vehicle noise becomes objectionable, isolate the noise to one area of the vehicle. Many noises that are reported as coming from the differential may actually originate at other sources. For example: tires, road surfaces, wheel bearings, engine, transmission, exhaust, or vehicle body. Differential noises are normally divided into two categories: gear noise or bearing noise. A thorough and careful inspection should be completed to determine the actual source of the noise. This should be done before axle disassembly and component inspection is initiated.

The mounting bushings help to dampen-out differential noise when correctly installed. Inspect to confirm that no metal contact exists between the differential case and the body. Metal-to-metal contact can result in a noise. This noise could be eliminated if the components were correctly installed and tightened. The complete isolation of noise to any one area -requires considerable expertise and experience.

All differentials produce noise to a certain extent. Slight differential noise at certain speeds or isolated situations are considered normal. Differential noise tends to peak at a variety of vehicle speeds. Differential noise is not always an indication of a problem.

TIRE NOISE
Tire noise is often mistaken for differential noise. Tires that are unbalanced, worn unevenly or are worn in a saw-tooth manner are usually noisy. They often produce a noise that appears to originate in the differential.

Tire noise changes with different road surfaces, but differential noise does not. Inflate all four tires with approximately 20 psi (138 kPa) more than the recommended inflation pressure (for test purposes only). This will alter noise caused by tires, but will not affect noise caused by the differential. Rear axle noise usually ceases when coasting at speeds less than 30 mph (48 km/h); however, tire noise continues, but at a lower frequency, as the speed is reduced.

After test has been completed lower tire pressure back to recommended pressure.

BEARING NOISE (DRIVE PINION GEAR SHAFT AND DIFFERENTIAL)
Defective or damaged bearings will normally produce a rough growl. This growl is constant in pitch and varies with the speed of the vehicle. Being aware of this will enable a technician to separate bearing noise from gear noise.

Drive pinion bearing noise that results from defective or damaged bearings can usually be identified by its constant, rough sound. The drive pinion front bearing is usually more pronounced during a coast condition. The drive pinion rear bearing noise is more pronounced during a drive condition. The drive pinion bearings are rotating at a higher rate of speed than the differential side bearings.

Side bearing noise will usually produce a constant, rough sound that is much lower in frequency than drive pinion bearing noise.

Bearing noise can best be detected by road testing the vehicle on a smooth road (black top). However, it is easy to mistake tire noise for bearing noise. If a doubt exists, the tire treads should be examined for irregularities that often causes a noise that resembles bearing noise.

FRONT WHEEL BEARING NOISE
Loose or rough front wheel bearings will cause noise that can be confused with rear axle noise; however, front wheel bearing noise does not change when it is compared during drive and coast conditions.

Front wheel bearings can easily be tested for noise by raising the vehicle and spinning the front wheels. Also shake the front wheels to determine if the bearings are loose.

BACKLASH CLUNKING NOISE
A loud clunk noise upon acceleration and deceleration could be caused by the following components (or in combination):
- Differential pinion mate gear shaft-to-differential case bore.
- Axle shaft splines-to-differential side gear splines.
- Differential side gear hub-to-differential case counter-bore.
- Differential side gears-to-pinion gears.
- Worn thrust washers.
- Excessive ring gear backlash.

Measure the clearances, inspect the components and replace as required (or, if applicable, adjust to specifications).

KNOCK AT LOW SPEEDS
Low speed knock is usually caused by either:
- Brinelled universal joint spider
- Excessive differential side gear hub-to-counterbore clearance
Inspect and replace the universal joint or the differential case and side gear, as applicable.

GEAR NOISE (DRIVE PINION AND RING GEAR)
Abnormal gear noise is rare and, if it exists, is usually caused by scoring on the ring gear and drive pinion. Scoring is the result of insufficient or incorrect lubricant in the carrier housing.

Abnormal gear noise can be noticed easily because it produces a cycling tone. This tone will be pronounced within a given speed range. The noise can occur during one or more of the following drive conditions:
- Drive
- Road load
- Float
- Coast

Abnormal gear noise usually tends to peak within a narrow vehicle speed range or ranges. It is usually more pronounced between 30 to 40 mph (48 to 64 km/h) and 50 to 60 mph (80 to 96 km/h). When objectionable gear noise occurs, note the driving conditions and the speed range.

ENGINE AND TRANSMISSION NOISE
Occasionally noise that appears to be from the differential is actually caused by the engine or transmission. To identify the true source of the noise, note the approximate vehicle speed and/or RPM when the noise is most noticeable. Stop the vehicle next to a flat brick or cement wall (this will help reflect the sound). Place the transmission in NEUTRAL. Then vary the engine RPM slowly up and down through the engine speed that corresponds to the RPM noted. If a similar noise is produced using this method, the noise is being caused by the engine or transmission, not the differential assembly.