Noise Diagnosis - Differential

Many noises reported as coming from the rear axle actually originate from other sources such as tires and wheels, road surfaces, wheel bearings, engine, transmission, muffler or body. A thorough and careful check should be made to determine the source of the noise before taking apart the rear axle. Noise which originates in other places cannot be corrected by adjustment or replacement of parts in the rear axle. Rear axle differential gears, like any other mechanical devices, are not absolutely quiet and should be accepted as being commercially quiet unless some abnormal noise is present.

To make a systematic check for axle noise under standard conditions, observe the following:
1. Check rear axle lubricant to ensure correct level, then drive vehicle far enough to thoroughly warm up rear axle lubricant.
2. Select a level asphalt road to reduce tire noise and body drumming.
3. Note speed at which noise occurs. Then stop vehicle and with the transmission range selector lever in "NEUTRAL", run engine slowly up and down through engine speeds, corresponding to vehicle speed at which noise was most pronounced, to determine if it is caused by exhaust muffler roar or other engine conditions.
4. Tire noise changes with different road surfaces, but rear axle noise does not. Temporarily inflating all tires to approximately 345 kPa (50 psi) for test purposes only, will alter noise caused by the 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). Tire noise continues, but with a lower tone as vehicle speed is reduced. Rear axle noise usually changes when comparing drive and coast, but tire noise remains about the same.
- Distinguish between tire noise and rear axle noise by noting if noise varies with various speeds or sudden acceleration and deceleration. Exhaust and axle noises show variations under these conditions while tire noise remains constant and is more noticeable at speeds of 32 to 48 km/h (20 to 30 mph). Check for tire noise by driving vehicle over smooth pavement or dirt roads (not gravel) with tires at normal pressure. If noise is caused by tires, it will noticeably change or disappear and appear again with changes in the road surface.
5. Loose or rough front wheel inner or outer bearings will cause a noise which may be confused with rear axle noise. Front wheel inner or outer bearing noise does not change when comparing drive and coast. Light application of the brake pedal while holding the vehicle speed steady will often cause front wheel inner and outer bearing noise to diminish, as this takes some weight off the bearing. Front wheel inner and outer bearings may be easily checked for noise by jacking up the tires and wheels and spinning them or by shaking the tires and wheels to determine if bearings are loose.
6. Rear suspension rubber bushings and spring insulators dampen out rear axle noise when correctly installed. Check that no metallic contact exists between the spring and spring opening in the underbody or between lower control arm bushings and underbody or axle housing brackets. Metal-to-metal contact at these points may result in telegraphing road noise and normal axle noise which would not be objectionable if dampened by bushings.