Noisy diff on Audi A3 - what's the fault?

We have a 2006 Audi A3 S line. Recently, there was a noise coming from the rear left wheel. We were advised that a new diff was needed, which the Audi garage completed, but the noise is still there.

The garage are now saying the diff was noisy and needed changing - but the fact remains the car is still making the same noise, which is worrying as the work was over £4500, potentially for nothing. What should we do?

Asked on 25 November 2014 by EmmaJF

Answered by Honest John
Presumably this is a quattro, otherwise it wouldn't have a diff to the rear wheels or a Haldex clutch. These drivetrains are very tyre sensitive so if there is any variation in tyre size, load, speed rating or tread depth between the tyres, that can lead to noises, wear and trouble.
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