+1 on Benny's comments. You're hearing a main bearing knock due to excessive clearance after enough time for the oil to drain away from the bearings- - - -usually overnight. Drop the oil pan and inspect some bearings, both rods and mains since you're in there already. I'd be surprised to find "new" bearings. You'll probably find some of them worn down to the point where the copper backing is showing. If the crankshaft is round and straight (not tapered) install new bearings and check the clearance with PlastiGauge. Low oil pressure with a stock pump usually means worn bearings. The high volume pump just tries to hide the problem by overcoming the leakage past a wide-clearance bearing a little better, that's why we use them on race engines that are built with extreme clearances for low rotating friction. "Lost" paperwork usually means a seller has something to hide!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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