www.b-quiet.com/brownbread.html
Its called brownbread.. It works as well as dynamat for a fraction fo the price....
Or you could also spray your vibrating pannels with asphalt bassed undercoating material or bedliner.. Look for the stuff that contains silicone.
Its very possible that the pannel itself vibrates.. When I bump moderately hard, the sides of my trunk expand..
Scary as hell..
When I'm playing a bass cd.. Its the apocalypse.. :o
Anyways the idea is to add weight (mass) to the pannel.