Rumbling from 1991 200 Turbo Quattro

["jonathan rudlin" <jrudlin@ets.org>]

I just acquired a 1991 200TQ that is making a very strange noise which my 
mechanic believes is normal--but I'm getting very tired of.  When I release 
the gas at above 4000 rpm, there's a strange noise from beneath the car.  
As far as I can tell, this happens whether or not the car is in motion.  My 
mechanic suggests that this is fairly common on the 20 valve turbo engines, 
and I'm hoping someone out there might know something more about it--and 
how to get rid of it.