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I have noticed a brief vibration between 750 and 800 RPMs when pulling out. Same thing happens if I rev to that rpm with brake on.
Now that I have my 500 mile break in period completed, I pulled out a bit more aggressively, and noticed the same vibration (through the floor/seat) at around 3800 rpm.
My Sister just got an '02 M6 Vert and it has the same condition. I noticed it when I drove it for the first time. I haven't had time to look into the reason or a fix but I know what you're talking about. And I haven't ever felt it happen in my car.
That does sound different than what I've seen...no grinding over here just some vibration through the floor board and trans. tunnel. I hope you have a good dealer to go to and they will do something about it without too much time spent trying to prove it exists.
I posted about this earlier, only mine comes and goes for no aparent reason. I can only assume the exhaust is loose or something. I am going to have mine looked at at my next oil change.