I can also add that droning, resonant vibrations *can* happen on a diesel
Vanagon.  Mine has one at 55-60 mph...I don't have a tach so I'm not sure
what that works out to RPM-wise. I very much doubt they did that from the
factory, so I suspect mine is caused by one or more of the following
problems I've noticed on my van, which I'll be correcting in the spring:

- Missing air intake snorkel
- Torn left-hand rubber exhaust mount
- Small exhaust leak at entrance to muffler

Generally worn-out engine mounts are also a possibility.  Mine is an '82
so it has the rubber bushings on the carrier bars.  I haven't ridden in a
Tiico so I don't know if I"m hearing the same thing, but the engine noise
seems twice as loud at 55 as it does at 50.  It seems all out of
proportion to the change in RPM between those speeds.
             _ _
  __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
 / _` | | | | | |  +-----------------------------------------------------
| (_| | |_| | | | @ cyberspace.org
 \__, |\__,_|_|_| "...we're not getting the creeps who knocked the
 |___/             buildings down.  We're getting the creeps whose
                   address we know." -- Maureen Dowd