First I was going to suggest checking your oil pressure sending unit, but then you said you heard the valves knocking that makes it a more serious issue. I'd look into it soon before driving it too much and possibly cause more serious problems. Have the oil pressure checked mechanically at the sending unit (install a mechanical oil pressure gauge and see how much pressure you get when you first run engine after being cold). This should give you an idea. Sorry I can't help you more on this. I am sure some qualified tech out there can give you additional help. For now, just keep your eyes on the oil pressure gauge while driving. don't want to drive on no oil pressure or your engine will seize and you'll be looking at new engine!