Me too. I thought I had gotten a defective one when I first drove it. But then eventually, as other ppl got 545i's, I drove them and realized that they too do the same thing. As OP said, however, it's really only noticeable when you rev in neutral though - not something that you do very often. Even less so if your car happens to be automatic.

 

Motor mount is a good chance-if the motor is moving on revving-get someone to rev it and watch the engine-it would also do this during driving but you may notice less when the tranny is engaged providing more support.