It could be one of the cats...also, it could the plastic piece that covers the top arm on the inside of the car. You need to move the top to service position and look at them. Porsche uses a lot of plastic and they will rattle.
Old Hippie Young Heart
2000 S/3.2 Liter/Tiptronic/Boxster S Sport Package/Cruise Control/Slate Grey Metallic
Red Special Leather Interior/Red Floor Mats/Red Hand Painted Instrument Dials/Roll Bar/Windstop
Small Carbon Package/Leather Wrap Carbon Wheel/Center Console Exterior Color/Alum Carbon Shift Knob
AM/FM Radio w/CD Player & Changer/Digital Sound Package/18" Turbo Wheels/Wheel Caps w/Colored Crest
My front engine mount did the vibe at 3200-ish, but only on deceleration. I put a new one in last spring and its fine now...
I never had a vibration when accelerating though with my bad front engine mount. My guess would be because the engine torque tightened it up.