My '87 Cutlass (Daily Driver) has been making this rattling noise for probably a year now, it goes away with rpms. I have, for sometime dismissed this as maybe the timing chain is so stretched, it is contacting the block somehow. Now I have my doubts that that is the issue.

The noise has gotten a little worse over the course of the past 4 months. I just cannot seem to trace it to the source... The car runs great, has good power and gets driven a lot!

Question, what else could this be, I really cannot believe it is crank, rod, piston, valvetrain or internal engine related. I am now leaning toward a heat riser or something... has anyone else ever run across this?

Car is 307 4bbl, 6+ years ago it was removed from all of the computer controls (78 403 Q-jet and basic HEI distributor)

Any and all thoughts are appreciated.