Gnome with a hammer, and a rather large hammer, trapped inside your engine block..

Take a long handled screwdriver, a wooden dowel, or your mechanics stethoscope and press it against your ear and various bits of the engine like the valve cover, the cylinder head, and along the oil pan..

It will be loudest near the source of the noise.

Odds are (from here) rod knock.

 

Pull spark plug wires one at a time to isolate it to a certain rod.