Rattle at certain rpm (timing chain??)

[bcar240]

My '93 S13 (since I bought it 5 years ago) has always made a reasonably quiet but random ticking/slapping sound at idle, I've had a couple mechanics tell me it was nothing to worry about by itself. Recently it started making the sound constantly between ~2800 and 3400 rpm when the engine is warmed up. I started noticing it when I left off to shift and as the revs fell through this range it would make the slapping sound like crazy and then stop. I am guessing the sound is probably the timing chain and maybe indicates a problem with one of the tensioners. Does this sound about right or are there any other things it could be or diagnostic procedures to try.