Is it a knock or a tap?
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The knocking sounds as though it's coming from the OHC.
This would be a lose valve.
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I also feel the knock in a part just above the oil pan that appears to go to the oil pump

This sounds more like a rod knock. My Sister's '95 Runner had a knock one on start up that went away once warm, though it finally became constant, so she traded it in, rather than fixing it.

To deterimine where the knock is comming from pick up a stethoscope, and listen to the engine.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed