3000 rpm Vibration

[97 Boxster]

I know there has been alot of discussions on the board here about the 3k vibration and what that vibration comes from. My vibration comes at around 3k rpms and is between 69-73mph when I feel it in the steering wheel.

I had wondered if it was the wheels, shocks, tires or the engine mount.

So I decided to give a little test and I may be way off here so correct me if I am wrong. I would get up to 73mph at around 3k and feel the vibration/shake and then put my car in neutral and let the rpms drop and the vibration goes away when the rpms are down and my speed is still in that 69-73 range. This should be telling me that it is the motor mount correct?? Am I missing something here? If it was the wheels I would still feel it with the car in gear or not right??

Please help me with this. Thanks to all.