PhilD, This is my second drivers side mirror also :-(
I think GM's thoughts were when they first designed them was "what the hell it's just a mirror" and didn't put a lot of design thought into them.
With the arm being at an angle from the mounting/pivot point and the loose tol. they used, I think the mirror head weight would allow it to bounce around more and loosen things up causing the vibration to get worse.
The new design like DRTYFN has is straight up from the mounting on the door and the mirror head pivots instead of the arm and mirror like ours.
I bet the folding motor is in the head on the new design also instead of in the mounting bracket, just a thought :-)
Also, I found this
http://www.automotive.eaton.com/prod...powerfold.html while working on the leaking mirror motor problem I had and the one on the bottom looks real close to the one in ours. Mine had the 203 number on it which tells me I got the old motor design, if Eaton is the mfg of our mirror motors. The only possible help the 204 would be is maybe a more reliable folding motor but would not be any help on the vibration problem.
P.S. I did e-mail them to try and get some info on the motor, Doubt that I will ever hear from them but I can say I tried :-)