Road/wind/engine noise is a problem in our older Buses.  I want to get a 
louder stereo, so that I can be real deaf! ;)

I have considered placing sound-deadening pads in various locations inside
of my bus.  J.C. Whitney  sells them, and so does the Eastwood Company.
They are self-adhesive, thin, and can be trimmed to fit wherever you want
them.  The engine compartment could be lined with these pads, like a Bug,
and more pads can be placed inside of the Bus above the engine compartment.
Sound insulation can also be stuck to the inside of the doorskins and roof
to help dampen wind noise.  If you frequently drive your Bus alone, or with
one passenger, hanging a curtain just behind the front seats may also help.
The underside of the van can be undercoated.  The entire van can be
carpeted.  

All of these measures may help, but consider that the VW Bus/Vanagon has
many windows.  How can one limit noise from them?  Air-cooled engines are
inherently noisier than water-cooled engines because they do not have the
coolant-filled passages that dampen engine noise.  Furthermore, most of the
air-cooled Buses that I see/hear on the road have some degree of muffler
problems. 

IMHO - No matter what, the air-cooled variety of VW van never be made as
quiet as your standard, American-made, conversion van.  One can do things to 
limit the amount of noise in the passenger compartment, but a certain level
of noise must be tolerated. 

Wolf