A product I've not seen mentioned in this thread is________.  I don't know
the name of it.  But we used it when we built a very nice IOR racing
sailboat, a while back,  to insulate the motor box.  This  racing yacht's
Isuzu diesel motor was quite noisy and sat right dead center in the boat in
the middle of the main salon under the dining table (for
"mass-centralization" important in racing sailboats)  The material we used
was a closed cell foam on one side, some kind of lead cloth and then a layer
of vapor barrier that faced towards the open inside of the motor box.  We
bonded it on with 3-M adhesive.  This stuff..wow, it really reduced the
noise from that diesel auxiliary motor so that you could talk in a normal
voice inside there with just that soundproofing and a few mm of plywood
between you and the clattery diesel motor.
  I guess you might find this material at a marine supply house online..I
will be looking, as my van has a very noisy 'spot' in it's rpm
range..probably a combo of vibration and harmonics from the tranny coming
through the rear 'bulkhead' under the fold down seat.. It occurs right
around 60mph in my 5th gear, where I often find myself driving.. I think
this heavier material, with a good bond to the larger panels of the van,
would negate some of the harmonic vibrations, as well as just killing the
sound waves.
  Don Hanson