Dude.....
Just yesterday I got my windshield replaced by Safelite in my ST. It had cracks and was pitted. Tech finished the job (mobile done at my work). I walk out four hours later. I get on the car (it is a 113 degrees out) and start the car. Turn the AC on full blast and wham....the dash literally starts vibrating to the point I can't have conversation on the phone.
The vibration gets worse the faster the blower motor spins. Needless to say, I get annoyed and ended up pulling the under side of the dash apart. Pull the whole glove compartment out, and finally get access to the blower motor.
There was a large piece of $%^*ing paper towel, that had made a home in the blades of the blower motor. I figure when the tech didn't put together the outside cowl, and cleaned the windshield and dropped the paper towel in there. Then I started the car and turned on the AC, wham it was sucked down into the fan. When it would come up to speed it would unbalance the blower motor, and the harmonics and vibrations would resonate through the dash. Grrr.....
So I rigged up a long screwdriver with some two sided tape and fished the damn thing out. Blower is happy now, but my time was wasted and Safelite will be getting a call.