Oil pressure drops when idling and knock from main bearing.

The problem isn't with the oil pump. The oil pumps job is to move oil around the engine. What the problem is, the main and rod bearings are so worn out that it can't create oil pressure anymore. That's why you have a knock in the bottom end of the engine. The tighter the clearances are at the main/rod bearings, the better the oil pressure you'll have. The cam bearings also play a roll in producing oil pressure.

Well yes a match is one option.
When you were replacing the rod bearings did you check the main bearings too ? My guess is a worn out main bearing.
Once you get up to speed the crank is not in such a strain as when you are accelerating. And the worn main bearings can cause the oil pressure to drop when the oil thins out at operating temp.

Knocking in engine and low oil pressure usually means bad main bearings. May have been caused by bad oil pump or just worn out bearings due to high mileage. If this is problem needs engine or rebuild old engine.

first you will have to say what the oil pressure is cold, idle and 2000 rpms and then hot idle and 2000 rpms before a decision to advise about a rebuild can be made
there are many reasons oil pressure can drop and not all require a rebuild

No that is not exactly true, worn main engine crank bearings will drop the oil pressure as it leaks out the gap, but worn oil pumps will also cause the bearings to wear and then that makes the oil press drop problem worse. The bottom line here is that if you have an engine knock it is to late, the engine must be rebuilt and the oil pump is always replaced at that time.

Could be two things, your oil pump pick tube is clog, oil pan would have to be drop to check, or your camshaft bearing worn causing low oil pressure,I would not say piston rod bearing or main bearing, cause there's no knock noise. You will have better oil pressure when cold due to oil much thicker. so I would forget about a bad oil pump.

Sounds like it could be low on oil PRESSURE. Or possibly bad bottom end (main bearings). A lot of times when the clearences are excessive in the main bearings, oil pressure will drop considerably once the oil is hot and has a lower viscosity. Sorry about the caps, just wanted to clarify the difference.

Does the oil light come on or the knock occur during the engine warm up after a cold start?
There could be a couple of problems causing the issue. You could have an oil pressure pump or pump pressure regulator sticking. I would have a mechanical oil pressure gauge installed and verify oil pressure at idle and 2500 rpms to make sure oil pressure is in normal ranges.
You could also have excessive piston rod bearing, crankshaft bearing or cam shaft bearing wear that will cause oil pressure to drop too low to prevent the rod knock your hearing at 2500 rpm.
You said a knock and not a ticking noise,so I don't believe a lifter is the issue at 2500 rpm.
Start with the oil pressure check and go from there.
Let me know.
Regards,

OOO not good sounds like some of the head gasket got stuck somewhere, or the gauge or sender is bad or got knocked around when repairing the head gasket. I would check that first. If that checks out will need to drop the pan to see if something got stuck in the pickup tube,might as well change the pump while you are there. Or worst case some thing wrong with gasket job impeading oil flow back to bottom of the engine. Was the oil and filter changed ?if so pull out filter and see if an chunks are stuck in it now. And change oil and filter again. Sorry that was out of order I had a long day

Sorry to say it sounds like the motor is gone. Higher oil pressure when the oil is cold is normal. But, when the engine gets to operating temp and the pressure bottoms out and the engine knocks you have big problems. If the mileage is very high (150,000) or more your bearings are so worn that the oil can't fill the void between the surfaces and thus the low oil pressure.