Heres a few ideas....
As said, check the torque converter bolts. They will rattle if the engine is under very little load or on the rev down.
Are all your rocker arms in the correct positions? Is it possible that you have them out of order? Intake rockers in the exhaust position or vice versa? This will cause the pushrods to be off center and maybe increase the lash.
Physical interference with the engine or exhaust to the body or other obstruction?
Worn out motor mounts causing a metal to metal contact?