This site has a great deal of info on the subject.

http://www.shagadelic.org/VW/info/gmbulley-soundproof/

I have used butyl roofing for soundproofing in the front doors and under the
front and rear seats and coated the wheel wells front and rear with the
fibreglass-reinforced roofing compound. It has made it a great deal
quieter.  I want to do all the doors, but haven't made time yet.

The wheel well trick has done a lot to filter out road noise.  That and the
Yokohama H4s tires.

On 10/18/06, Royston, Jerry <gmr@obpa.usda.gov> wrote:
>
> I was wondering if any of you have done any soundproofing of your van or
> camper and if so what your experience has been.  Is it worth the effort
> and expense?  What would be the least expensive material to use that
> offers good results?  It seems if you could make the van a bit quieter
> it would greatly enhance the driving pleasure.
>
> Gerry
>

--
Jake
1984 Vanagon GL
1986 Westy Weekender "Dixie"
www.crescentbeachguitar.com