I have a 3 litre quattro sport avant with an increasingly worrying rumble. Its a low frequency grumble when under load that isn't related to wheel speed or revs.
It started in 5th and 6th gears but is now throughout the box but it only seems to be once the car is warm.
It doesn't seem to affect performance but its loud enough that you lift off at about 4000 rpm so I havent been brave enough to try to drive through it!
Its definately at the front end and not the prop bearing noise that some people have suggested.