[Synchro909]

I'd start exactly where you did - checking the middle main bearing. That not being the cause of the noise, I'd move on and check the other mains and the big ends. Is the knock worse on initial start up, then reduce as the oil comes through. If it is like that, I lean towards a main bearing. If one of those is loose, the oil drains out of it quickly after you shut it down so the bearing is running fairly dry till the oil pump gets oil up into the gallery and it runs down to the bearing. If it is a big end, the rod is dipping oil from the word go so you won't have that delay. Check them all and while you are in there, you will see if there is anything disastrous like broken babbit.
It sure sounds like a bearing to me.