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How to check and confirm a differential whining (hissing) noise

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Method for confirming differential whining or hissing noise

BEFORE YOU START:
The below works on any 2WD car with a normal differential that is not self locking, and with any manual or automatic gearbox.
Do not try the below on a permanent AWD / 4WD vehicle, or a car with self-locking differential.
A differential that lost its oil, or whose bearings are worn, will typically emit a high-pitched humming noise (whining) that increases when accelerating at very low and low speed and remain audible when driving on a flat road at constant speed and on high gear.
In case of doubt, use the below method to make sure 100% of your diagnostic.
  • Simply lift the car on the side where you suspect the whining noise, or any side if you're not sure the differential noise is coming from the left or right bearing.
  • Turn on the engine and put the gear on "Neutral".
  • Make sure your wheel rotates freely and that no noise comes out when rotating it by hand, except from the friction of the brakes.
  • With the car still lifted on one side, put your gearbox on "Drive".
  • The wheel will start rotating and you can then hear the whining noise of your differential very clearly.
  • If not, try the same to the other side.

What does a differential whining (hissing) noise mean? 

In case the differential noise is confirmed, proceed as follows:
Check the level of oil in your differential, add oil if low, and check if the noise vanishes.
Look for the cause of the oil leak and fix it.
If the level of lubricant is not low, or if the noise remains after filling with oil, then your differential bearings are worn and are causing the high-pitched humming; the differential needs to be replaced.
You can also buy a specific car noise analyser tool, which we did not try however:

See also...