Really keen to hear what the result is on this one as I am having the exact same issue. No one I seems to know what the issue is as i have looked at it and had many others and when I try replicate it murphys law it doesnt happen.
Straight line for me its fine and pulls well. But pulling away and turning I get the thud,knocking noise. Craigcull the best way to replicate the noise is put 2 passengers in the rear seats then go for a drive. You will probably find it does it a lot more often.
I was told change diff oil which I did mine was quite low but nothing was leaking out as people suggest mainly from the shafts. Little improvement but still knocking.
Was also told the rear anti roll bar can affect this some how so, considering that mine was corroded anyway. On with the RS4 rear roll bar fitted. Again I thought the knocking had stopped but I dont have such luck.
The only thing I have not done that has been suggested is to replace the tyres. Some say that cheap tyres especially non OEM tyres like chinese tyres which are a very hard compound tend to do this I read a thread about a guy explaining the same issue and 56 pages later on a thread turned out it was his tyres.
On the other hand 80% of audi tech members I know state that it takes a lot to damage the rear differentials on these cars. And almost all 3 people have said they dont think its the diff. And one particular place I took the car into stated that they had a customer with the same issue replaced his diff and still did the same thing. Which in one hand makes me think it is possible or on the other was his diff just a refurb one and had the same issue? Who knows.
Personally for me I think there is a possibility that tyres could play a major factor. Am I sold on the fact that it is going to cost £600 to eliminate the noise?? not really. Especially as I still have 6mm tread all around dont really want to be replacing tyres just yet.
My only other possible solution is it is something to do with the propshaft. I know there is a centre rubber mount that if you look down the tunnel just above your exhaust you will notice a Ring type rubber that houses around the propshaft some say that is the cause and they get weak and soft over time but it is a entire propshaft out job to get to this rubber sadly if it were to be replaced that is. Which to me sounds more plausable.
The one thing I can also suggest Craigcull is to get the car back up on the ramp and do one of 3 things. Check that rubber ring around the propshaft for play, check your diff fluid and check if it is full if not fill it up to the filler line and that no fluid is dripping out of the diff. Lastly lift the car off the ground enough so that the rear wheels are free and make sure the car is in "N" or neutral then try this place both hands on the propshaft and turn it clock wise and take particular notice on how long it takes for the diff to actually turn the rear wheels if that makes sense. This is considered back lash and again is another possible reason why I think on mine that its the propshaft because when I turn mine you can hear a knock and the wheels dont turn almost instantly its like there is a delay "or play" in the box itself. Which im told is not normal. So that is all I can suggest on my end I really hope you find the solution.
As it annoys me to DEATH!!
Just to comment on the mounts mention in the above posts. People have tried these especially the bloke from the 56 page thread I spoke of and he said it actually make the noise a lot worse because it seemed even thou it stiffened up everything it made it more pronounced if that makes sense. Kinda like the same vibration effect that members complain about when fitting a snub mount.
Not sure if i can post from other audi forum sites. Might get shot by the forum "snipers"
But here goes ,
wierd juddering when on full lock
Have a read there. Thats just one of the complaints and they in particular talk about the 3.0tdi quattro
