My 2003 Passat GLX 4Motion developed two bad rattles in the dash: one was a standard metallic rattle in the center of the dash, the other was a squeaking rubbing, almost creaking, sound on the passenger side.
The dealer fixed the center dash rattle by putting in a plastic support for the Monsoon radio. He said the back of the radio was hitting up against something in the dash.
The right side squeaky rattle was fixed by remounting a crossmember in the dash, which was rubbing up against the interior of the dash itself.
Just thought I'd pass this along in case anyone else has the same problem(s).