What's causing a droning sound from my Volskwagen Golf TDI at 70mph?

I have not long taken delivery of a 2005 Volkswagen Golf TDI 4-motion with 67,000 miles that also has a most awful droning noise at approximately 2000rpm or 68-70 mph. It's as if the vehicle shouldn't be in the highest gear: sixth, but a lower one. I get the feeling that it's coming from the rear of my car but could easily be the exhaust, but not the tyres which are brand new Michelin Alpin3. Luckily I haven't had to drive for too long a journey yet but am not looking forwards to it when it comes.

Asked on 21 September 2013 by JR, Llandrindod Wells

Answered by Honest John
With a 4-motion AWD system all four tyres need to be as close as possible to the same circumference or the system may interpret the disparity as slippage and engage AWD. But if all the tyres are new the reason could be running cold weather Michelin Alpin 3s in hot weather, or a problem with the rear differential.
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