I had the same problem for some time, and it turned out to be the exhaust banging against the body or bumper, can't remember which one. �But only when starting the van.

Simple problem and caused no damage to anything.

Anthony

________________________________
 From: Steven Shelton <shelton4@GMAIL.COM>
To: vanagon@GERRY.VANAGON.COM 
Sent: Thursday, March 27, 2014 8:01 AM
Subject: Thunk! When Starting
 

Lately, about 1/3 to 1/2 the time i start my 1985 1.9L, I hear and feel a
loud THUNK! from the rear.� It runs OK after the thunk.� I thought it was
most likely motor mount(s), but yesterday i was looking under the van and
saw a wet spot from coolant dripping from somewhere on the driver's side of
the engine.� Last night I came up with the unpleasant theory that the� two
could be related; the coolant could be leaking into the cylinder as well as
outside, and the thunk could be momentary hydro-lock.� Is this a
possibility?� It's a VW rebuilt engine wit 54,000 miles on it.