Forgive the cut and paste, but good info nevertheless . . . .
Diagnosing Driveline Problems
1. Whirring noise only while decelerating at any or all speeds is most likely caused by bad pinion bearings or loose pinion bearing preload, and almost never by bad ring and pinion gears.
2. A howl or whine during acceleration over a small or large speed range is usually caused by worn ring and pinion gears or improper gear set up.
3. Rumbling or whirring at speeds over about 20 mph can be caused by worn carrier bearings. The noise may change while turning.
4. Regular clunking every few feet may indicate broken ring or pinion gears.
5. Banging or clunking only on corners can be caused by broken spider gears, lack of sufficient positraction lubrication, or worn positraction clutches.
6. Rumble while turning may indicate bad wheel bearings.
7. A steady vibration that increases with the vehicle’s speed can be caused by worn u-joints or an out of balance driveshaft.
8. Clunking only when starting to move or getting on and off the gas might be loose yokes, bad u-joints or worn transfer case or transmission parts
Several situations can create ring-and-pinion noise. If the gears have been quiet and begin to howl, they are probably worn or wearing. If the gears howl during deceleration only, it’s possible that the pinion-bearing preload has loosened. Howling under acceleration at all speeds indicates that something in the differential -- gears, pinion or carrier bearings -- has worn or no longer keeps the gear alignment correct. If the gears howl while accelerating over a certain speed range, but not all speeds, it’s likely that the gears are worn due to lubrication failure or overloading. When a newly installed gear set howls, suspect the design or setup.
Most rear axle failures are relatively simple to locate and correct, although rear axle noise is a little more difficult to diagnose and make the necessary repairs. The most essential part of rear axle service is proper diagnosis of the problem.
All rear axles are noisy to a certain degree. Axles can be noisy if they are not properly adjusted or lack lubrication. Usually, when new improperly set gears are noisy, the disturbing noise can be “adjusted out”.
If the axle noise complaint is made within the first 1,500 miles (2413 km), and the gears are not scored due to lack of lubrication, the gear adjustment should be checked and corrected if necessary. Poor tooth pattern, incorrect drive gear backlash and loose pinion nuts are the primary causes of gear noise in new cars. Regardless of what you’ve heard to the contrary, noisy gears will not get quieter with added mileage ... they will stay the same or get worse.
Slight axle noise heard only at certain speeds or under remote conditions must be considered normal. Axle noise tends to “peak” at varying speeds and the noise is not always indicative of trouble in the axle.
If noise is present in an objectionable form, loud or at all speeds, an effort should be made to isolate the noise as being in one particular unit of the vehicle. Many noises, reported as coming from the rear axle actually originate from other sources such as tires, road surfaces, wheel bearings, engine, transmission, exhaust, propeller shaft vibration, universal joint noise or body drumming. A thorough and careful check should be made to determine the source of the noise before any disassembly and teardown of the rear axle is attempted.
Axle noises normally fall into two categories; gear noise and bearing noise. The complete isolation of noise in any one unit requires considerable skill and previous experience. Eliminating certain type noises often baffle even the most experienced personnel. Often such practices as raising tire pressures to eliminate tire noise, listening for the noise at varying speeds under different load conditions such as; drive, float and coast, and under certain highway conditions, will aid in detecting axle shaft bearing noise.
To make a good diagnostic check for rear axle noise a thorough road test is necessary. Select a level smooth blacktop road. This will reduce tire noise and body drumming. Drive the car far enough to thoroughly warm up the axle to normal operating temperature. Drive the car and note speed at which noise occurs. Then stop car and, with clutch disengaged or automatic transmission in neutral, run engine slowly up and down through engine speeds, corresponding to car speed at which noise was most pronounced, to determine if it is caused by exhaust roar, or other engine conditions. Repeat, while engaging and disengaging clutch (transmission in neutral), to determine if noise can only be isolated by removing propeller shaft and operating transmission in high.
Tire noise is often mistaken for rear axle noise even though the noisy tires may be located on the front wheels. Tires that are unbalanced or worn unevenly or have surfaces of non-skid type design, or worn in a saw tooth fashion are usually noisy and often produce noises that seem to originate in the rear axle. Tire noise changes with different road surfaces, but rear axle noise does not. Inflate all tires to approximately 50 pounds pressure (345 kPa) for test purposes only.) This will materially alter noise caused by tires, but will not affect noise caused by rear axle. Rear axle noise usually ceases when coasting at speeds under 30 miles per hour (48 km/h); however, tire noise continues, but with lower tone, as car 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 differential noise by noting if noise varies with various speeds or sudden acceleration and deceleration; exhaust and axle noise show variations under these conditions while tire noise remains constant and is more pronounced at speeds of 20 to 30 miles per hour (32 to 48 km/h). Further check for tire noise by driving car over smooth pavements or dirt roads (not gravel) with tire at normal pressure. If noise is caused by tires, it will noticeably change or disappear and reappear with changes in road surface.
Front Wheel Bearing Noise
Loose or rough front wheel bearings will cause noise which may be confused with rear axle noises; however, front wheel bearing noise does not change when comparing drive and coast. Light application of brake while holding car speed steady will often cause wheel bearing noise to diminish, as this takes some weight off the bearing. Front wheel bearings may be easily checked for noise by jacking up the wheels and spinning them, also by shaking wheels to determine if bearings are loose. Rear suspension rubber bushings and spring insulators help to dampen out rear axle noise when properly and correctly installed. Check to see that no metallic interference exists between the springs and spring hangers, shackles or “U” bolts. Metal to metal contact at those points may result in telegraphing road noise and normal axle noise which would not be objectionable if properly installed and tightened to specifications.
The differential side gears and pinions are not a source of noise during normal driving, since the mesh condition changes too slowly to be heard. They will however, emit an audible howl during high rate differential action, as when one rear wheel spins while the other is stationary. This audible howl is not abnormal, but should be regarded as a warning since prolonged high rate differential action can result in extensive damage from seizure of differential components. Abnormal gear noise can be recognized easily because it produces a cycling tone and will be very pronounced through the speed range in which is occurs. Gear noise may be developed under one or more of the following conditions, “drive”, “road load”, “float” or “coast”. Gear noise usually tends to peak in a narrow speed range or ranges. Gear noise is more prominent between 30 to 40 mph (48 to 64 km/h) and 50 to 60 mph (80 to 96 km/h). Abnormal gear noise is quite rare and if present it usually originates from scoring of the ring and drive pinion gear as a result of insufficient or improper lubrication of the axle assembly. If the gear noise is on coast at all speeds, check for a loose pinion nut and readjust per this manual if necessary. When objectionable axle noise is heard, note the driving condition and speed range. Remove the housing cover. Perform a tooth contact pattern check to determine if the best possible pattern has been obtained. If pattern is found to be unacceptable, reshim and adjust to obtain the best possible tooth pattern. If after readjustment noise still persists, replace with new gear set.
A close examination of the rear axle assembly prior to disassembly can often reveal valuable information as to the extent and type of repairs or adjustment necessary. This information coupled with the road test results will provide a basis for determining the degree of disassembly required. Since the most frequent causes of axle noise are improper backlash or differential bearing preload, or both, a few simple adjustments may be all that is necessary to correct the complaint. Therefore, before disassembly the following check should be made; drive gear and pinion backlash, pinion bearing preload, and tooth contact pattern and these results recorded and analyzed. It is felt that these measurements and their results will aid you in making the necessary repairs to the axle assembly.
Bearing Noise (Drive Pinion and Differential)
Defective or damaged bearings generally produce a rough growl or grating sound, that is constant in pitch and varies with the speed of the vehicle. This fact will allow you to diagnose between bearing noise and gear noise.
Drive pinion bearing noise resulting from defective or damaged bearings can usually be identified by a constant rough sound. Front pinion bearing noise is usually most pronounced on “coast”, whereby rear pinion bearing is loudest on “drive”. Pinion bearings are rotating at a higher rate of speed than the differential side bearings or the axle shaft bearings. These particular noises can be picked up best by road testing the vehicle in question on a smooth road (black top). However, extreme caution should be taken not to confuse tire noise with bearing or gear noise. If doubt should exist tire treads should be examined for irregularities that will often produce such noise.
Differential bearing noise will usually produce a constant rough tone which is much slower than the noise caused by the pinion bearings.
Rear Wheel Bearing Noise
Defective or damaged rear wheel bearings produce a vibration or growl which continues with car coasting and transmission in neutral. A brinneled rear wheel bearing causes a whirring noise. Spalled rear wheel bearings normally produce a noise similar to a growl, created from either flaked or pitted rollers or bearings races. Unless the damage is severe, rear axle bearing noise is seldom heard above 30 mph (48 km/h). If the noise in question cannot be isolated an inspection of bearings will be necessary.
Knock at Low Speeds
Low speed knock is usually caused by brinneled universal joints or differential side gear hub to counterbore clearance being too great. The knock caused by excessive side gear hub to counterbore clearance will be on coast only. Inspect and replace universal joint or differential case and side gear as required.
An audible metallic clunk accompanied by a bump can occur when the foot is abruptly lifted from the accelerator pedal, or when the pedal is abruptly depressed from its free position, while driving in the 30-55 mph. (48-89 km/h) range. An objectionable level of clunk and bump can be helped by reducing the amount of axial looseness at the differential side gears, using select thickness thrust washers. The procedure is covered in the Differential Reconditioning Section.
Engine and Transmission Noise
Sometimes noises which seem to originate in the rear axle are actually that of the engine or transmission. To diagnose which unit is actually causing the noise, observe the approximate vehicle speed and conditions under which the noise is most pronounced; stop the vehicle in a quiet place to avoid any interfering noises. With engine running and transmission in neutral, run engine slowly up and down through engine speeds corresponding to approximate car speed at which the noise was most pronounced. If a similar noise is produced in this manner it usually can be assumed that the noise was caused by the engine or transmission and not by the rear axle.