Joel had some great tips on sound deadening.  here's one more.

go to any store like Target, Wal-Mart, etc. and buy a queen or king 
size 'egg-crate' orthopedic pad ya s'posed to sleep on.  they are 
about $12-$15.  if the interior panels are off, just cut pieces of 
this stuff and glue it to the metal panel, (possibly ON TOP of 3M 
Undercoating, using the undercoating as an adhesive-- haven't tried 
that) with 3M Super Spray Adhesive.  I don't remember the 3M part 
number but on the can it says it is compatible with open-cell foam 
rubber.  This egg-crate shaped stuff dampens the drumming resonance 
and acts like acoustical tile.

I'm not sure of the fire safety of this stuff.  Original insulation 
in the vanagon Westy was fiberglass wool bats.  The foam pad does 
have a California fire acceptance tag on it.  I also tried burning 
some with a match before installing it behind my hot fridge.  It will 
ignite, but the flaming part extinguishes when the match is removed.  
It seemed safe enough to me so I did install it in my van.  
Fiberglass insulation would have more fire resistance but i HATE 
working with it... just cut the foam rubber with a scissors!

-dan