Hi Mark/All,

it's the hollow drive shaft ringing in 'torsional vibes'.  Some slop in the
drive train lets it spring back after it has wound up. Sounds like an old
school bus. You wind it up while in gear, clutching fast lets it snap back.
Could be rubber vib'n damper, not avail from VW, but IS available from BMW,
I have the part# somewhere. It is more likely a bit of wear in the R&P
gearsets, try to check by hand if concerned.  Doesn't hurt but is a reminder
of the age of your van.

bye, Tim