The volume inside a Vanagon can be helped by a very careful assessment of
how the motor, transmission and exhaust are connecting with the chassis.
It's not widely understood, even by exhaust pros, how a little it takes to
get a lot of noise inside. It's similar to using something against the
motor and against your ear to listen....

I just replaced my muffler...same mounts but the guy who welded for me
snugged up a "redundant safety-only" exhaust hanger...one that was
purposely slack... rubber even....  the noise inside was bad.... I knew
exactly what he did, and when I slacked-off on that....again quiet!

  Motor mounts, transmission mounts and also the floor is a big source of
noise....