new motor has gremlin knocking

[nvrsatisfied]

a buddy of mine put together an ht 383 crate motor. Started it up, gave it a break in time for the cam and so on. took it for a couple of short test drives with open headers. he then heard a slight knock, thinking because he had no exhaust this caused the problem. he then got the exhaust put on and the noise is still there. he can't really pinpoint the knock. It's not a lifter knock and all of the rod bearings felt good. I could turn the engine over a full revoulution and didn't feel or hear anything hit. It does not knock all of the time and it definitely sounds like it's internal. the oil pressure reads great. he has taken an engine stethascope to get a better idea of where the noise is coming from and it helps but he still can not pin point it. he has dropped the pan twice and everything feels and looks fine, thought it was the flex plate and it turns out to be ok.......he has a mud event coming up and is wanting to get it going....but now is hesistant because of the noise.....ANY IDEAS!!!!!!!!