The rear differential distributes torque to the rear wheels in all-wheel or four-wheel drive trains. Regardless of whether you have a traditional solid axle rear diff or a hybrid with CV joints driving the wheels, noise from the rear end is always bad news.
Diagnosing clunking or whining noise with vibrations from the rear of your car is easy if it is front-wheel drive. It has to be the wheel bearings or braking system.
If it is a 4-wheel drive or an all-wheel drive, the rear differential and drive mechanism curveball the complexity.
Luckily, wheel bearings go awry sooner than the rear differential. There could be a chance your differential is OK, and your wheel bearings are the problem.
ProTip: Both noises sound the same. However, they are prominent under different scenarios hence easy to differentiate.
How to Tell the Difference Between Differential and Bearing Noises
You can run a couple of tests to tell if you are dealing with a damaged rear wheel bearing or a faulty differential.
The Wheel Wiggle Test
A wiggle check is a great way to inspect your wheel’s attachment integrity. While it can help you detect other problems like the front axle, it is easier to use when diagnosing wheel bearings in the rear.
- Suspend the car safely such that the rear wheels don’t touch the ground
- Secure the vehicle such that it won’t fall off the lift or jack stands
- Grab the wheel and wiggle it back and forth while observing the hub
- The hub shouldn’t allow massive back and forth movements or clunking noises
If it does, it means you have a damaged wheel bearing. This could be the cause of the noise in the rear.
How Doe the Noise React to Change in Speed and Load?
This is a great way to isolate the cause of clunks, whining, and grinding to your rear differential.
Wheel bearings will make noise directly related to wheel speed (your forward motion.) The pitch and frequency won’t change if you drop a gear for higher RPMs or when your engine is under heavier load driving uphill.
The differential is the problem if:
- The noise also seems to vary depending on engine RPM
- The noise gets worse when your vehicle is under heavy load (going uphill) and eases out as the load lightens (going downhill or when you ease off the accelerator without braking/actively reducing speed)
Steer from Side to Side as You Drive
Wheel bearing load varies depending on whether your vehicle is changing direction or what wheel is handling the most load at a given time.
You can simulate varied loads by changing directions by weaving side to side on a secluded driveway or unused road. The changes in direction shift weight and strain to different wheels.
If the noise changes as you weave about, chances are one or both of your wheel bearings are the problem.
Pre-Emptive Checks
If you can’t isolate the problem, you can pre-emptively check both the bearings and the rear differential to narrow down the problem by looking at other symptoms.
- Do you see any dents, oil leaks, and smears around the rear differential? Chances are it’s leaking and has no lubricant; hence the noise
- Is it a high mileage vehicle whose differential has never been serviced? If yes, have the differential looked at
- Inspect the wheel bearings manually to ensure they don’t wiggle and are well seated. They’re cheaper and easier to check than the differential.