Have you guys heard of this?

I have a 1973 Corvette that I‘m in the process of re-freshening. I have been working with a friend of mine and Corvette extraordinaire and to-date I am trilled with the car with one exception, I have a NOISE, an annoying one.
A few of the facts: the car is nice, very nice, as nice as a un-restored 33 year old car can be. I bought it from an older gent that stated that the car had been in his family for a long time. I moth balled the L82 (wanted to keep the original pieces unmolested) and opted for a nice 383, AFR head, Weisko Pistons, MSD... lots of good hard parts but everything on the mild side (again, a driver). In addition, we did a full Vette breaks suspension & break kit, short shafts ... all the good stuff for a driver. The package is noting short of amazing to drive, great car for the first ~100 miles and THEN, the car started to make a noise, somewhat intermittent (comes & goes??). The noise is a strong rattle, like a diesel. I have checked the valves, (sounds like bad/loose lifter), everything looks OK. During the original rebuild, we installed a new a B&M 1800 stall, & rebuilt the THM 400. The noise comes in ~ as the stall engages maybe a clue?? It has been suggested that the converter is the culprit, so I have purchased an additional new B&M and am going to replace as soon as I get a chance.
My question, have any of you heard of this problem before?? I could use some well thought out advice.

Thanks for any advise!!

 

Possibly stuck lifter (if hydraulic), too much timing, pre-ignition. Those are the easy ones. Others are bearing related. Ouch![]

 

What is your ignition timing set on? What grade of fuel you running? What is the engine temp when the clatter happens? Sounds like detonation. Might have to use some octane boost if you have a lot of compression.