It was the front engine mount. It was toast. replaced and problem completely fixed.

Mikstew,

My 06 Cayman S is doing exactly as you described in your original post. First time I heard it I was in 6th gear 70mph and put it on the floor, heard a rattle.

I took it to my Porsche mechanic, he has it now. No metal in oil, no leak, just the rattle under a load at low RPM.

Your outcome would be helpful.

Thank you in advance.
Dustin