I had one of the tinfoil style ally exhaust heat-shields partially break loose on another Audi I had and this give the exact same symptoms... tinny rattling at tick over rpm and then vanished at anything over 1200rpm... I think the local independent I used at the time said it was unrepairable as it had sheared the securing screw off in the bodywork due to corrosion, so they added a ty-wrap or self tapper and it didn't rattle again..... could be something along those lines anyway.
