Also check the plastic under panel covering the body, not the engine. Where it bolts, you get cracks and breaks, just the right speed and it becomes noticeable.
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
it's an '01 with 34k miles. I will check further underneath, but it sounds like a fluttering noise almost. And its only when the engine is is in low gear at higher rpms, like down a hill coasting in 3rd or in 2nd driving.