Has anyone tried anything like Dynamat? I would imagine you could put some on the mounting plate with some success. There is a cheaper alternative in the roofing section of a place like Home Depot or Lowes. Basically, it's an alumninum sheet with a sticky, tar backing, and it's fairly heavy for it's size. It comes in a 6" high roll and looks similar to
this (it's a little different, though, but you get the idea). I used this to deaden vibrations in cars when installing car audio equipment, and it's very effective (and much cheaper than Dynamat).