85 with a newly rebuilt engine.   Seems to run fine at speed, but at
idle is behaving very weirdly.  Been fiddling with the idle adjuster
screw, and can't seem to find a setting that lets it idle when cold,
w/o surging upon deceleration at 1500 rpm when warm.  Pretty sure the
throttle closed switch is correctly adjusted, timing is good, etc.

The latest thing that has me worried is a bizare "clunk/zap" sound
coming from the airbox.   It happens intermittently when the idle is
dropping below 900.  Dissappears completely if you goose the throttle
to get it above 900rpm.    As near as I can tell, the sound is caused
by a "pop" that is reverberating through the throttle body, AFM, and
airbox.  If you put your hand on it you can feel it happening.
This is a brand new rebuild engine, and this is a sound that I've
never heard on a vanagon before.

Could this be caused by a broken/stuck/misadjusted intake valve?  (so
that when the cylinder fires, pressure is going back up the intake
manifold?)  If so, why would it only happen when the engine is
lugging at idle (say, 750 rpm or so?)