oil pressure drop!!

[Bolo]

My sr has been worrying me lately. At idle, the oil pressure used to be 20 psi. Now it has dropped to about 16 psi at idle. But at higher rpm's the pressure is still good. Anyone know what could've caused this drop? Is my oil pump going out? Could it be rod bearings wearing out and blocking some oil passages?

I also noticed this grinding noise everytime I do a cold start. It sounds like metal to metal contact but it only lasts for a couple seconds. After the engine is fully warmed up and I start it again, it doesn't make that noise.

Any ideas??