Why does the oil pressure suddenly drop from 40psi to 10psi.

What is your question?

I have the exact same problem with my 2006 2500HD with 6.0 engine. Driving down the interstate and the oil pressure went from the normal 40psi to around 10 psi on the gauge. Only reason I noticed it is because the rocker arms or lifters started clicking. I took half the damn engine apart to replace the oil pump and still only 10 psi and lifters rattling. Not sure what to do next. Unfortunately the Chilton repair manual for this truck is useless. Incorrect parts shown etc. Also, no drain on the radiator. Whose brainstorm was that at GMC. Any ideas on the what else the problem may be?