That's pretty normal - it's a resonance issue. The stock stereo has a bunch of boom at whatever frequency the mirror rattles at. My stock stereo does the same thing.
Having rented *lots* of rental cars, I can say that a good percentage of cars do this, especially if they have a boomy set of front speakers.
Of course, any car with good subs will be able to do the same thing - I used to be able to darn near tell which note was being played by looking at the mirror in my old Tacoma. You could just look at it as a point of pride, that the lowly stock stereo can vibrate the mirror
