This may have nothing to do with your knock but here is what mine is. I have a new Avant engine. When I got it it had the stub in it instead of the vacuum pump. When I took the stub out the hex connecting rod came with it, pulling it out of the little nylon retainer that sits on top of the oil pump. When I put it back in there was no way to get the rod fitted right on the keeper because it bottoms out on the oil pump before the retainer groove makes contact. When the oil gets warm it taps like crazy; cold it is not so bad. It's also not so bad after a fresho oil change. I know it is this because when the stub is in there it hardly taps. Since the vacuum pump has a cam in it it alternates pressure on/pressure off of the drive shaft making it tap. There is no way to fix this short of taking the pan off, that I can see, so I'm living with it. I went the route of cracking lines, swapping injectors, checking harmonic balancer, etc., etc., until I found what it was. I don't think it is serious - just annoying. Maybe there is some additive or something a guy could use. Good luck.
Randy
2006 GMC 2500HD Duramax Allison Crew