Rebuilt 340 knocking sound

Throttle needs to be wired open, all plugs pulled out, and cranked off of starter relay until pressure stops climbing on the gauge. But overall, feeling bad for you on this one... What was oil pressure before it started going away? There are a lot of things that can cause what you're talking about, from dirty assembly (by far the most common) to clearances being okay but things not being as round as they should be to a cam bearing walking out or to an internal oil gallery plug coming out.