Damping to exitation of parasitic modes of

forget everything except altering the Q - you will first need to determine the freq.s involved (tho any decent car stereo dampening material should be helpful)

I've tryed using sorbothane but have had problems with the substance degrading. It also ruins wood finishes!

For micro vibrations best to use constrained layer damping as well as very hard cones that will allow such unpleasantries to escape the system. I also am a fan of natural cork for damping and isolating printed circuit boards, capacitors, those pesky large transformers and gulp, fuses/fuse holders. Sorbothane not so much. I've had trouble with the sound degrading. 😛

+1 on constrained layer damping and not Sorbothane.

Dave

So Jeff, your saying you believe in "Sympathetic Vibration" and "Compassionate Resonance"? I'm unsure about the resonance and have found no way as of yet to prove anything positive about those vibrations.
   Clue me in.
    As far as the Sorbothane goes you may have used too much in mass of the wrong durometer. I've also had problems with it not loading to spec. Besides that, It smells funny.