Q: low oil pressure on 1999 Chevrolet Silverado 1500

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My oil pressure drops to 25 pounds after I drive for awhile. After I shut the engine off and let set over night, the pressure is good when I start it.
(2) Answers
(4) Comments
Here is the oil pressure spec for your Silverado -
42 kPa (6 psig) minimum, at 1,000 RPM
125 kPa (18 psig) minimum, at 2,000 RPM
166 kPa (24 psig) minimum, at 4,000 RPM
Depending on the engine rpm your oil pressure may not be to far off.
That is not good. The knocking noise could be from a worn engine part with the worn part causing the loss of oil pressure. I would suggest you have a qualified technician listen to your warm engine. They may be able to give you some idea what is causing the knocking noise.
at 70 miles my gage usually reads between 50 & 60, but after I drive for awhile it drops between 20 & 30, and I get a knock on the drivers side of the engine.

take your valve covers off take a clotheshanger and clean your port holes out thay tend to clog
on the oil pressure specification it a misprint called your chev. dealships it from 37psi-60psi
you need to replaced your oil pump unless you replace the oil sending unit. the specification for oil pressures is from 37-50psi