"I have Che's muffler on my 02 S and it makes a rattling sound as well. Porsche dealership said it was coming from inside the muffler, maybe a loose baffle."
I would guess the muffler is rattling, but I traced a rattle to the TTP headers that I have. The bends in a few of the pipes are very close together and at lower RPM's, they can rattle. Also, I just got a Dansk Sport muffler and there is some rattling that comes out until the engine warms up.
Ed