Got an 87 K5 Blazer with a 305, oil pressure guage has always jumped between 0 and 40 once in a while since i got the truck 5 years ago. Now there is a loud knocking metallic clicking noise that sounds like its coming from the oil pan. The rocker arms have all been adjusted and were oiling themselves while running. Could this be a oil pump going bad?

 

Have you verified oil pressure with a mechanical gauge?