I have had my new (old) audi for a few months.
1998 a4 1.8T 5 speed
There seems to be a rattle/rumble similar to the sound and slight feel of driving over rumble strip on the edge of the road (http://upload.wikimedia.org/wikipedi...mble_Strip.jpg)
The sound and shaking seems to be coming from the centre (under between the gear stick and armrest) of the car
(Worrying sound - YouTube) I kind of had my hand over the mic, but you can here the difference between the sound i am trying to describe and the engine rev. half way though.
This mainly happens shortly after driving though puddles/driving in the rain or when there's lots of slush on the road.
I have taken it to a couple of garages' locally but none of them have been able to reproduce the problem.
Right now my best guess is that its an issue with the center differential (I ran 1 band new tire and 3 baud ones for ~16,000km, might have caused an issue, or may have been a problem I inherited)