1990 4.0 Troubles

I just picked up a 1990 with a 4.0 that has since been rebuilt.
Started tearing down the engine today and could not find what was causing my knocking sound. There are no visible gouges, no rods are bent, valves seem fine, I cannot for the life of me figure out what the problem is. The block seems fine because there was plenty of coolant (no leaks). The oil seemed near new(no shavings). Any one have any ideas. The guy i purchased it from said he was driving up a hill, then lost power and pulled over and got towed home. It still ran but had a knock and didnt have much power. There was no oil pressure and I dont know what else to check. Any pointers?

Piston slap? Also check the flex plate for cracks/loose.

Was it rebuilt before the knock or after?

It was rebuilt and ran for about 60K before this kid had his mishap on the freeway. When i went to buy it the kid said it had a transmission problem. He also didn't notice that the oil pressure gauge did not work so he wasnt the brightest.

I have pulled the oil pan and have checked everything while cranking by hand. I can't see anything and will tear it down completely tomorrow.

An oil pump wouldn't cause this sound would it? It was a knocking sound that would increase as you gave it more throttle.

After some more research on the forum, it appears that piston skirt could be bad...any experiences?