I have a similar vibration between 2800 and 3000 rpm.
Mostly on decelleration.
I've learned that it can be the dual mass flywheel that make this vibration.
Do a search on all P-forums and there will be alot of reading.
If you change the enginemount to a new one you might not feel the vibration in the car, but it will still exist!.
I think this is some kind of designproblem in the engine that Porsche don't wan't to talk about due to their reputation
