well i thought it was gone but it wasn't but i did actually find it.
..if you take the door panel off and look through the door with the window up you can see the 4 mounting bolts for the mirror but one bolt is not accessable through the factory holes in the door...its sits behind the glass of the window, only one!
that bolt was tight but not all the way...the washer rattled! you have to get a 10mm wrench and squeeze your hand behind the glass and tighten it by hand until the waher doesnt move anymore. funny thing is the passenger side makes the same noise
so i have to do that side also. but the drivers side is quiet now!! hope this helps anyone that has the same dilema and can't narrow it down. fricken GM factory workers!