So is it crappy repro transmission covers ( 727 small block 71) that are causing the insane vibrations as I drive or is something else the culprit?

I replaced the missing cover , tightened everything down, no missing bolts an drove the car , it was fine.
Getting off the parkway and merging into traffic on road, I start hearing insane vibrations, so bad I think the transmission is about to blow. I gingerly drove it to my mechanic down the road (was going for an inspection).
He put it on lift and started car, and it was definitely the cover making the god awful noise. All bolts were secure.
Is the metal on the new covers just cheap and thin?
My other 71 with same cover doesn't vibrate at all. Not sure if it is a repro or not.