why does my 454 sound like a diesel?

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.

I've been through most of the usual suspects on the engine -- ignition timing, valve noise, etc. I really seems like the sound is coming from under the car but I've never heard a sound like that come from a clutch/trans. The fact that it only happens at lower revs (below 2000rpm or so) and only moderate load (can't hear it when I get on it) makes me wonder if it's something rattling around until centrifical force pins it down.

one of the springs in the clutch hub. that dampen is broke???

Piston slap maybe?

worked at a shop were this mustang had a new ford crate engine and a new tremec trans. ran but the guy had this sound that went rigth thru the trans . sounded like it was getting hit with a hammer .so i looked at it for him he was thinking it was the trans but he had the wrong fireing order. the give away cold tub on the header and it really did not sound like it had a dead hole.you can check were the sound is coming from with a broom handle the type that unscrew or a very long screw driver:whacked:

the 454 is external balanced do you have a 454 fly wheel????:whacked:

This might not help. But my uncle had that same similar sound. what it turned out to be was when he come off the line the torque and twist of the car made the exhaust rub against the sway bar. it took forever to find this. just a suggestion

Could be bearings in the trans, cluster gear chipped, maybe.Even pinion noise can travel up the driveshaft and sound like the engine.