Thanks for the quick reply.
I thought to check the flywheel carefully for cracks, but it looks good.
My description of it knocking when it gets ready to shift may have been a little misleading. When I sit there in neutral and bring the rpm up to about 2500 the knock begins. And it sounds like a rod knocking too.
Maybe when I get the rods off the crank I will see some kind of damage. This is not my first time to work on a Chevy, but it is the most serious problem I have encountered.
In other words, I am in deeper than ever before and I still don't have the pistons out yet.