Humming Noise from the Rear of Your Car?

Raise the vehicle with a jack and jack stands and feel the wheel for excessive play or stiffness. It should move in and out with minimum play, but it should not feel excessively loose or frozen stuck.

Remove the wheel for a closer look at more parts that could be causing a humming noise.

More on how to jack a vehicle up

Make sure the brake rotor is cool to touch. Feel if any heat is radiating or if it’s warm. You don’t want to touch a burning hot rotor.

Look at top of brake caliper for debris and dirt accumulating. If that gets on brake pad or rotor, it will cause an issue. The debris could also cause the burning smell we smelt in the video featured in this article. Fluid burning off due to heat of the brakes will also cause a burning smell.

Take the brake parts apart for a closer look. Remove the brake caliper from the caliper bracket.
Inspect the area of the brake caliper where brake fluid should be. Remove brake pads to access the caliper pistons.

The caliper pistons have a rubber boot. Check the boots to see if any fluid is leaking, which means the piston is leaking brake fluid, and that can lead to a build up of debris on the caliper.

Check the brake caliper for excessive debris that might be collecting on the brake pads. If brake pads have debris on them, they will also need to be replaced.

More on inspecting the brake caliper

See if the brake pads have any dirt or debris that might be sourced to a leak. If the brake pads or shoes are swollen from contamination, they’ll end up overheating and will need to be replaced.

More on how to inspect the brake pads

Inspect the brake rotor for hot spots. Hot spots are scored sections on the rotor that can cause discoloring, a brake pulsation, and brakes overheating.

Remove the brake rotor and check the rotor for gear oil that may have leaked onto it. We have a ridge on the rotor that’s swollen from pressing against the backing plate. The rotor could be hitting against backing plate and causing noise.

In our example the rotor has several cracks. Even one crack means you’ll need to replace brake rotor.

Check for discoloration possibly from overheating.

More on inspecting the brake rotor

In this example, we had a rear differential axle leak. The gear oil might be leaking onto the brake shoes, which is unsafe makes for unsafe driving conditions. The axle seal may have separated, causing it to leak fluid.

The differential in the video featured in this article has a little bit different of a set up than most rear differentials. This seal is for a free-floating differential, meaning the hub in the back can “free-float” a little, so the seal has an inner portion that can spin while the outer portion remains still. There’s a spring in between the inner portion and the rubber seal. For this application, check the spring and make sure it’s seated perfectly.

If the seal is damaged, broken, or looks misplaced like it fell out, that will reduce it’s ability to apply pressure and that can cause an axle seal leak.

More on checking differential leaks

Inspect the bearings located behind the axle seal. The bearings can also look different depending on the application, but for a howling noise, check the roller bearings. Look for scratching or discoloration like blue, which are signs the bearings are overheated and have wear.

Wiggle the bearings and check for movement. Generally a little movement is fine, but if there’s excessive movement like it wants to separate or come loose and pop out, it will need a replacement.

Typically, there will be multiple bearings, consisting or an inner and outer bearing. In this example the outer bearing is in the open, but on a different differential there might be a seal with a bearing located behind it that’s harder to see. Generally, for the outer bearing you’ll have to remove the seal and give it a quick inspection. There will also be an inner bearing, which compared to the outer bearing is harder to inspect without taking apart the differential.

Inspect the brake shoes for any damage or wear. If the brake shoes are covered in gear oil, they’re contaminated and you can only service them by replacing them. If you need to replace the brake shoes, we also recommend replacing the hardware at the same time.

Inspect this area thoroughly. We also found gear oil along the area of the backing plate, and on further inspection this area gave an appearance of being ground into something, as if a part has been riding up and hitting against the plate.

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In the example in our video the seal on backside of the wheel hub looks grounded up and scored. The backside had been hitting the backing plate near seal, causing damage and making a howling sound.

More on how to inspect a wheel hub