Rear Axle Noise: Isolating the Problem

How to diagnose and isolate a rear axle noise problem

Noise and clatter from the rear axle of a car or light truck isn't the norm. If you hear strange sounds emanating from the back end of your vehicle, it really is something to be concerned with. On a brighter note, plenty of noise you first think is coming from the rear axle assembly can actually be attributed to other causes. Even better, the majority of problems that actually originate inside the rear end can be diagnosed by listening to the sounds while driving. There is a caveat: When turning your attention to rear axle noise and isolating the problem, use an open stretch of road with no traffic.

Rear axle assembly noise can often be confused with the din of the driveshaft, tires, clutch, transmission and even the exhaust system. Luckily, there is a way to isolate the sound of the rear axle from these parts—and that’s by coasting the vehicle with the engine turned off. As you might have guessed, coasting with the engine off can be challenging. Keep in mind that the power steering won’t function and you’ll only have one or two opportunities to use the vacuum in the power brake booster before it is consumed (after that, you’ll have a high pedal pressure manual brake system). Never perform the coast-test down steep hills or at high speeds. Pick an open area (a big empty parking lot is perfect) and coast only for short distances at low speeds. Ten to 15 miles per hour should prove suitable. More on testing later.

Before you begin a series of tests, you should first check the level of lube within the housing. In the vast majority of cases, fluid level is checked by removing the filler plug. Rear axle lube should be within "finger distance" of the filler plug opening and should be clean and fresh (keep in mind that rear axle lube, without question, has a rather "unforgettable odor," even when fresh). Rear axle lube is often specific for the rear axle assembly (for example, open versus limited slip assemblies). Because of that, always be sure to use the fluid recommended by the manufacturer.

Watch for leaks surrounding the drain plug or at the axle seals found on the ends of the axle housings. Lubrication loss at the axle seals can usually be seen on the inside of the wheels or tires. It's sometimes confused with brake fluid. If you see a large amount of fluid on the inner surfaces of the back wheels and tires, double-check the brake system.

At this point, you can test and troubleshoot the vehicle for driveline noise. For this purpose, four separate tests are recommended: one in a conventional driving mode (under light throttle), another in a cruising condition, one while coasting and finally, one test while the speeds change. In all cases, turn off all accessories such as the radio, AC/heat and so on. In all four tests, it's wise to check for noises first with the windows closed then with them open. Here's the rundown:

Drive Test

Drive the vehicle on a section of a straight and level roadway. Use light throttle pressure (sufficient to allow for gradually increasing vehicle speed). This test will allow you to determine if there is a noise under load. This should also help determine what sounds are constant under load conditions. A solid whining or humming noise may be due to a loss of lubrication, the use of an incorrect lubricant or improper mesh of the gears (which is basically a rear end setup issue). In this case, the noise can also be attributed to dry (lacking lubrication for whatever reason) or worn wheel bearings.

Cruise Test

Drive the vehicle at a constant speed, again on a section of straight and level roadway. If the noise is created by components other than the rear end or the driveline, it should become apparent with this test procedure. Engine noise, exhaust noise and tire noise can (obviously) all create unwanted sounds. This din is often mistaken for excessive driveline noise. The purpose of the cruise test is to help isolate these sounds, since they are easier to detect and eliminate.

Alternating Speeds

This is a combination of the acceleration and cruise test methods above. Begin by driving the vehicle at a constant speed then accelerate lightly (low road speeds are best in this case, and there is no need to exceed 10-20 mph). Take your foot off the gas pedal suddenly. This will create a sudden transfer of torque within the driveline and rear end. Worn bearings inside the rear-end assembly or in the wheel bearings or excessive clearance (wear) between gears will produce a sharp "clunking" noise. Worn driveshaft universal joints can also produce this sound. Before assuming it is a problem with the rear end, inspect the driveshaft u-joints.

Coasting Test

Operate the vehicle at a constant speed then take your foot off the gas (release throttle pressure), allowing the vehicle speed to decrease. As pointed out above, if you're in a closed location (parking lot), try turning the vehicle "off" during the coast test. Coasting takes the torque load off the drivetrain and effectively puts the vehicle weight on the backside of the ring gear. A constant noise (whine) during the coasting test often indicates damaged gear teeth or improper adjustment of bearings in the rear end.

There is another sound that can indicate rear end and driveline problems: You may encounter a sharp metallic click when shifting from a forward gear to reverse as well as a clicking when turning a corner. This noise indicates loose ring and pinion fasteners or perhaps a chipped tooth in the ring and pinion gears or spider gears. If the sound doesn't appear until you turn a corner, there's a good chance the problem is related to the rear axle spider (side) gears.

For a closer look at rear axle assemblies, check out the photo gallery and captions.

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