I also think it is maybe harmonics as the console that is arround the gearlever also starts to make a harmonic type of vibration sound when I hit these revs.
When you hold onto the gearlever while it is making these vibrations it is a very similar feeling like I explained above of the hammer drill that I feel on the gearlever. A more rrrrrrrr type of vibration on the gearlever.
(The description sounds stupid but that is the only way I can describe it).
My tailshaft was once out of balance and it made a totally different vibration in the back.
The vibration does not feel like when your wheels are at the front out of balance and the whole front starts to shake. It is more like a harmonic type of vibration. That is why I wanted to know if you other guys have similar vibrations who's engines are not balanced but totally stock.
Maybe I must take it to my mechanic and show him and ask his advice.

gino
My exhaust is a straight 2,5 inch exhaust all the way to the back with one mufler at the back. No mufler in the centre.