Don't forget the obvious.
The first night I ever parked the PT in the driveway (not in the garage), someone tried taking off the performance wheels & tires, but didn't finish the job. I had the same vibration scenerio, called my dealer service rep. He reminded me to check for loose lugs before I went to a local dealer.
Yup! they gave up after trying to steal the wheels after loosen 4 of 5 locking lugs on one wheel.
Boy, was I lucky! (tipped him $20 the next time i saw him!)
