Update: pulled the inspection cover and flexplate seems fine. Took it back to transmission guy who said the bracket holding the fuel lines was not bolted in place on the left side of the Bell housing when he removed the transmission (idiot engine installer did that). So he bolted it back in place and thought it was transmitted engine sounds to body amplifying them. He bent the bracket a bit and wedged some foam between bracket and body. Most of the odd rattle went away. So at the moment it appears cause was not a serious problem.
93 GMC 2500 pickup
4WD Xtra cab.
New Optimizer 10/14
Rebuilt Central Plains Diesel IP 10/14