What a coinkidink- this sounds uncannily like the exact same rattle that my recently acquired '02 base model has!
The rattle in question originates from the passenger side rear end of my car and gets gradually more pronounced as the car heats up. This led me to believe that a heat shield in the exhaust system was loose so I had an indie Porsche garage look into it.
Sadly, it turned out that the passenger side exhaust manifold catalytic converter was damaged and had to be replaced in order to eliminate the infernal and embarrassing rattling sound. As nasty as the phantom rattle sounded, the $2,021 repair bill that's coming my way offends my ears even more!
Place a mechanic's stethoscope on the passenger side exhaust manifold cat to determine if this is the culprit behind your car's rattling sound. For your wallet's sake, I hope a heat shield is loose...