I have had some success at reducing noise levels in my 84 Westfalia.
I bought 6 sheets (app. 4x6x0.5") of closed cell neoprene sold for sound
insulation at a cost of $120.  I used this to line much of the body panels
and doors and also the rear cargo area and under the rear seat.  In the
last two areas I doubled the thickness in most areas and sandwiched a layer
of aluminum foil between them (on advice of a manual about sound proofing
aircraft that came with the foam).  I also put two layers on the underside
of the engine cover (you have to make some cut outs to avoid putting pressue 
on some components). I have also lined the floors and also the front part
of the poptop roof that is exposed when the roof is down (this has the 
advantage of preventing condensation on the roof as well as keeping in
heat).  In addition to the foam I have packed fibreglass insulation into
as many spaces as I can reach easily (much cheaper than foam but probably not
as effective for sound insulation).  Finaly I have used canned foam to fill
in some of the major body pillars.

The result?

Well it neither sounds or looks like a mercedes! But it is a bit quieter.
Engine noise is very much reduced and doors sound "dead" when closed.
I am sure it is also alot warmer/colder as a result of the extra heat 
insulation.  It is still not quiet though.  I am thinking of putting some
sound insulation in the wheel wells since alot of sound is coming from the
tires.  Any more ideas?

--
David Shepherdson