[boxstershorts]

This is a mechanical engineers (not an experience Porsche mechanic) crack at this question:

Vibration responses happen at different frequencies caused by part rotating at different speeds. Knowing a what speed (or frequency) you hear the vibration isn't normally much help because you would need an in depth structural analysis to figure it out.

One trick that could help you at least hear the noise better is to drive beside the wall of a building. That will reflect the sound much better and hopefully help you figure it out. I do this when I am test driving a car, it really highlights problems. Or if you can reproduce the vibration at low speed, have someone sit in the rear boot.

Adding mass or stiffening up the part will change the frequency of vibration. If you think it's the heat shield, you can tape it up (as a test) to see if the frequency changes. If it does, you found the problem. It might be trial and error until you find the culprit.