I installed some Dynamat in the doors of my Vanagon to help
eliminate spurious audio system sound waves.  It is an excellent
adhesive sound deadener.  When installed on the inside door skin it
also eliminates almost all of the hollow door closing sound you get
when you slam the doors shut with the windows open.  Double duty
product... how bout that...  It is expensive, but hey, you're all
used to paying VW prices anyway, so you won't mind.  Any good car
audio store will stock it.

Brian.