I know you have used the screw driver method to listen to the engine and noise is from dist area.but have you used an actual mechanic stethoscope? If not, Place place along the block on the sides of each cylinder, , Each head bolt, , Each oil pan bolt,, on top of each rocker stud, Starter bolt, rear main part of oil pan with engine running, Oil galley plug next to dist, I can tell a definite difference between how I can hear something between a screwdriver and a mechanic stethoscope. I use the one from Harbor freight and it works much much better than a screw driver.
Hurts to say this, since the noise is internal and not valve train, Even if you find the noise, it will have to come back out....
Wishing you the best.
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