The 'drone' your Tiico has is most likely caused by the left engine support.
Read on:

Back when the Vanagon diesel was introduced her in 1982, the engine support
bars were mounted with rubber ends. These ends allowed the bars to vibrate
at certain rpms, and VW added a strap between the two supports thinking that
if both vibrated in the same direction, then the vibration would be
cancelled out. It didn't work all that well, so in 1983, VW changed the
engine support bars by mounting the ends solidly, which is the way they were
then produced from that day on. This worked well to quell the vibration. For
SOME strange reason, the bean-counters at Remtronic (the supplier of the
Tiico kit TO Tiico) is using left-over left support bars from 1982
production. This setup, with one old-style bar and one new-style bar, I have
found to vibrate around the 3400 rpm range. (Almost all Tiico 2.0 liter
engines are the roughest-running 2.0s I have ever seen). By replacing or
better (cheaper) yet modifying the left support to have solid ends such as
the right support has will decrease the amount of harmonic vibration sent
through the body and to your backside, as now the rubbers engine mounts
(between the supports and the engine's aluminum castings) will now do their
job better. I've modified a few Tiicos this way, all are much smoother now.

Sound deadening around the airbox is a good idea, and tossing the
Tiico-supplied restrictive snorkel in favor of the later Vanagon snorkel
tube with insulated flex-section will also help. Also, make sure the muffler
is mounted very tightly in it's brackets, modify the brackets if needed for
a tighter fit.

Good luck!

Karl Mullendore
Westy Ventures

As with many, a clear drone noise starts at 3,300 rpm and continues upward.
The noise only occurs under load - ie when I run at 3,300 rpm standing
still, drone is not heard. As for tone, most of my auto aware friends
hearing the sound think it is either exhaust or air intake related.