Rebuilt 340 knocking sound

First off, low oil pressure and cylinder cranking pressure are TOTALLY DIFFERENT AND UNRELATED THINGS.

Here's what I'm observing, of importance.

1. You were driving it. All was well. You started hearing noise, and low oil pressure.
2. Checked oil and it's silvery.

Bearings gone. Cam, crank, rods, any of them. Silvery oil equals bearing material, most likely. Pull the engine, find the rod or main bearing that's faulty and repair. Check clearances etc and property lube.

Compression has nothing to do with any of this. 150 is maybe a bit low but not awful.