This is interesting Andrew. Yes, I think it is something like this. It varies a little but after about 10-15 minutes it is smoother. Although, since time has passed since I had the last motor mount changed, it is getting worse. I now feel the slight vibration a little longer and more continuously. Not sure how it would be the alternator even.
It does feel like a motor mount, but one mechanic said if it was the motor mount, it would vibrate all the time. With your theory, this would explain the varying degrees of vibration.