I am experiencing a strong vibration when I start up and run the car until the engine warms up. Even after it is warm, it is still present, worse when the a/c compressor starts up, but not as bad as when the car is cold. I just had a new serpentine belt put on, and the problem started immediately after that,no such problem before that. I then had new lower motor mounts put in, and that did not cure the problem. A mechanic suspects a heat shield is touching somewhere, but cannot find anything out of place. The car runs great but for the bad vibration. Any help is appreciated.