clunk between 2000-3500 rpm

[nde264 at lulu.it.northwestern.edu]

My 89 90q has developed a clunk/rattle that sounds like it's coming from the
engine compartment.  While sitting in my car at idle, it is intermittent and
sounds as if a shroud or cover is loose and rattling around.  Increasing the
engine speed increases the frequency of the rattle.  The rattle does not happen
every (x) seconds though.  Yesterday, I started my car, and after an abnormally
long time cranking (I have a new FP in the trunk), the rattle was much louder.
I revved the engine to see what effect that would have.  Frequency got higher,
then the noise quieted down.  I am able to get the engine to make the same
rattling noise by revving over 3000 and letting the engine drop to idle.  As
the revs drop from about 3500 to 2000 rpm, the rattle is at its loudest, and
seems to be almost gone at idle.  I've not yet been poking and prodding at
parts in/under the engine compartment while the car is running.

Any ideas as to what this could be?  Am I hitting a resonance of some sort?
Engine mounts?

TIA,

Nathan Engelbert
89 90q