I have a low freq knocking, about 6 to 10 knocks, very loud on startup until oil pressure comes up. Knocking quiets down and engine runs well with no knocking but then returns at idle as oil pressure decreases after warmup, but not as loud as on initial startup. I replaced main crank and rod brgs, rebuilt oil pump, replaced timing gears and chain, all lifters and water pump. Compression on all cylinders is about the same and within normal specs. No improvement in level of knocking. Do I need to suspect a bad rod, pins or cam bearings?
If you still have knocking, it means that there is too much clearance somewhere. While you replaced the main crank and rod bearings - did you have the crankshaft ground and the new bearings matched the new sizes or did you just put new bearings around the old crank (not the best idea).
A possible different solution is to fit a small electric pump to build up the oil pressure before starting the engine, but finding the real cause and correcting that is better.