A3/S3 quattro propshaft vibration damper.I am posting this, mainly for anyone who searches for the actual item, although I know at times diagnosis of the problem is not straight forward and frequently taken for a number of other things.
I am talking about A3/S3 quattros, 2004-2007 including likes of TDI quattros, 8p models, possibly newer ones and maybe even all quattros but not sure as I don't really have enough knowledge but all must have similar dampers and certainly the 04-07 ones seem to get this issue fairly commonly. Basically if you get a vibration through the drivetrain at around 2/2500 rpm then it's likely to be this part at fault.
The genuine part is made by ZF Engineering under the name Lemforder and retails from Audi for £270 ish plus vat (at time of writing). It's a big round part (sometimes referred to as a doughnut) that fits between the rear of the propshaft and the Haldex unit (there is also a front one but they are protected by the engine cover and rarely need replaced) and it's meant to reduce vibration but as it gets older the rubber hardens and movement in the rubber and between the inner and outer rings lessens, it becomes less effective and you get unpleasant vibrations as you are driving, usually when accelerating fairly hard.
Some Audi garages (from what I have read) will have you replace the entire propshaft at a cost of over £900, so beware, as others will supply this part only (Part No. 1KO 521 307A) if asked.
I have now found a supplier who actually brought them in from Germany at my initial request, he will do the same for you or try to source whatever alternative part you need. He did this one for me for approximately half the price of Audi, same OEM part, same maker. Here is the link to their website; http://www.allgermanparts.co.uk/
Jason Hunter is the guy I have dealt with and have to say I have nothing but praise. He only has limited numbers at the moment so be quick if you need one but I am sure he will get one for you if you can wait a bit too, which is much better than ECP etc who just say it's "unavailable".
Hope others find this helpful, if you have any queries I can help with send me a PM, probably should say I have no link to this company or vested interest in it. I was just fed up of paying over the odds for my parts, been stung too many times before and took on the mission to get this one particular part at a decent/honest price