Wow, that narrows it down =), but seriously. This knocking I am referring to, sounds quite litterally like a hard physical knock on the floorboard, right between the center console and the passenger seat. It is strong enough to stir change around in the center console. I first thought it was the shaft bearings in the tcase, but it is pretty solid, very minimal movement. I have to shake the bejesus out of it to get it to move at all *and it is very little*. I am sure it is not the cooling fan, I have heard that before. You may be right on the flexplate, but that sounds pretty difficult to figure out without disassembly, but it is in the same area of question. Do you know of any aftermarket slip yoke eliminator kits?
Don't take life to seriously, nobody gets out alive.