GM-GMC: 1994 Chev 350 Oil Pressure, chev 350, dissapears

Question
1994 Chev 350 Silverado - at first start up and while idling the oil pressure drops down to little or nothing and the engine knocks very loud.  Usually a small rev will make the pressure come up for a while and the knock dissapears.  When driving the pressure is usually alright, however sometimes it does drop and there is a noticable knock and some loss of power.  Any ideas what would cause this?  Oil is filled to correct level and with proper grade.  Changed as scheduled and otherwise maintained quite well.

Thanks,

Brad

Answer
Brad,

It could be any of the following:  

A partially plugged oil pump inlet screen.

A defective oil pump.

A defective oil pressure bypass valve.

A damaged camshaft and/or crankshaft bearing.

There is really no way to know without removing the necessary assemblies and looking for one of the above problems.