This evening will be the moment of truth, driving 150 miles and will be on the A31, M27, M3, M25 and A1, plenty of opportunity to see if the whine has gone.

I know the shudder sound is still present, it still does it almost daily at some point. Still not got to the bottom of that, could be bush related or perhaps transmission or engine mount related.

After driving 150 miles last night, I can confirm the whine is still present but seems quieter.

The shudder is still present as well, this seems to always sound the same.

I also drove 80 miles yesterday with passengers, so we all had a good listen. Whine is definitely in the rear, but its location is inconclusive.

The whine is quiet loud on deceleration, for example lift off at 70mph and you can hear it, and the frequency drops as the car slows down. Apply throttle and the sound gets quieter, speed up and the sound goes all together.

The whine will stop completely if you move from Drive to Neutral thus disengaging the transmission. Revving the engine and the noise does not return, put the car back in drive and the sound returns.

When we arrived at our destination yesterday, we had a feel of the temperature of all road wheels. Not sure but its possible that one wheel was warmer than the other at the back of the car.

Still suspecting rear diff or wheel bearing at this stage.

Your whine is different to mine. Mine is purely engine RPM so would return in N when revving. Yours sounds more like rear diff, so maybe you could change the fluid. It's very easy to do, easier than changing engine oil. Mine was jet black like cocacola when it was replaced.

You can use infrared thermometer to check tyre temperatures. I have done it to dial in TOE without going for wheel alignment.

I also posted this on audiworld to get a view from the other side of the pond.

Audi have a TSB for what this sounds like
http://ebookbrowsee.net/tsb-audi-a6-...pdf-d232161926

I wonder if this is my issue, and what could I do to get Audi to check, assuming the same TSB applies to the UK as well as USA.

Have a look to see if it is black as in the TSB, I looked and mine is silver, PR code FNL (tdi not listed on the US TSB.
My car was built ~ December 2005 and registered Feb 2006.
Good luck, I would try new oil before a diff swap though.

I've found a reasonably local diff and transmission specialist, I've buzzed them an email but not had a reply yet. If I dont here soon then I'll give them a call and see if they can help diagnose the rear end noise.
http://www.wimbornemotors.co.uk/axle...erentials.html

Well my car is still whining but I think I have found the source and its not diff. Managed to get a chance to sit in the back with someone else driving, tracked the source of the noise to the NS wheel bearing. The noise has got no worse or better since starting this thread so will need to keep an eye on it.

I too get a whining in the car on long journeys, it usually starts after about 30mins of driving and will require stopping somewhere convenient for a short break.

Its usually not long after setting off again before it returns.

I don't think I will ever be able to cure the problem, been with her for 11 years now so cant see her changing her ways in the foreseeable.

Good news on finding the source of yours though +++