What could be causing a transmission fault on my 2005 Volkswagen Touareg?

I have a 2005 Volkswagen Touareg 2.5TDI with 30,000 miles, which has developed a transmission fault. Moving slowly up a gradient, the transmission seems to wind up with a loud clicking from near-side rear. Volskwagen diagnostic checks reveal nothing and Volkswagen says it has had no reports of this fault. Possible centre diff problem? I would appreciate your advice.

Asked on 14 December 2013 by SB, Ramsey, IoM

Answered by Honest John
The most likely reason is a difference in circumference between the front and rear tyres that the automatic AWD system interprets as slippage.
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