[Obsessive]

"...continuous metallic knocking." That can't be good and can only get worse. I'd pull the pan, as you suggested. If you do not get to the source of the problem, don't give up. Pull the valve covers and start methodically removing valve train gear to inspect the lifters, springs, everything.

The piece is a real mystery. Do you have roller lifters? It might be one of the retainers that (otherwise) keep the lifters from rotating within the lifter bore. This kind of thing has happened to the best of us. Good luck!