I'm rebuilding a 71 Camaro SS with a 454 that came out of a 74 Chevy titan morothome. As long as I've had the car (6 months), it makes an annoying diesel-like rattling sound at lower-revs while I'm accellerating in first and second gears. I've messed with ignition timing and determined that it's not detonation (it only makes the sound in low gears). I thought it might be an exhaust leak of some kind but installing headers and new pipes didn't fix it. I didn't change the mufflers yet but the sound doesn't seem to be coming from that far back.

Could this be a clutch/tranny problem? It has a Saginaw 4-speed and, according to the prior owner, it has an OE clutch. I'm wondering if there's a problem with the throwout bearing rattling or something out of adjustment with the clutch disk or pressure plate.