I have an '85 Campmobile, automatic, 112K, and generally in very good
shape.  A disturbing event has begun to occur:  every so often, which is
about every 2 or three weeks, when I start it up a loud knock,
valve-like, but also similar to an exhaust leak, happens.  It gets
louder,then softer, without any particular event guiding it and then,
after I shut it down and restart it, it fades away not to return for a
few weeks.  I was baffled, took it to a VW mechanic who could not
duplicate the sound and said that it may be my valves sticking and
eventually this sound will not go away, until the inevitable valve job.
Sound familiar to anyone?  TIA.

Mark