THAT SUCKS! But.. you're upgrading, so it's not that
bad I suppose
We dislodged one of my buddy's Honda Accord's two mufflers coming back from Memphis. He ran over like a 2 by 4 piece of wood .. it's ratteled ever since (he's going to do a staight pipe this around instead of the Y-pipe).
My rattle is not what you described, and I think it's due to the fact that it only happens when I shift at a lower RPM...and sometimes I hear a rattle when I close my car door.
Post pics of the exchange process -- I would like to know (so I can attempt this on my bug at a later date).