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.