Are you saying the diesel sound is not knocking rods? Because that's what I always think of when I hear diesels, sounds like a few rods knocking....which makes sense with your low oil pressure story. I'm thinking putting some heavy weight oil or some special anti rod knock stuff in there?

Lifters make a ticking noise. Especially when starting up on high mileage engines. Rods knocking sound like 2 hammers hitting each other... cause that's basically what's happening, metal slamming into metal.