...............I've used a product found at the hardware store used as
for flashing when installing new windows.  ........
BTDT, have a couple of rolls of that stuff stuffed insice the doors,
sides, and slider of my Westy, also anywhere else that sounds like a
drum when you tap it.  Really does stick like hell to metal, not as well
on other surfaces like inside closests, etc.  Six inches wide and a 25
foot roll run about $13, a fraction of the price of what appears to be
the same thing with a different label on it calling it 'sound deandning
material'.  And it works......