He's got a diesel, so I think he's got a rubber band instead of a chain. I'd be looking at stuff like the belt tensioner pulley bearing, water pump, injection pump, p/s pump, alternator, a/c pulley clutch, and pray it's not the water pump or engine internals.

A mechanics stethoscope is cheap and mandatory for finding engine noises. It's a doc's stethoscope with a metal rod connected to a small metal diaphragm that's connected to the earpieces, and you'll be amazed at what you can hear. Sound travels well thru the metal of the engine, and gets a lot louder at the source. Watch the moving parts proximity to mortal flesh and tools...

A poor man's mechsteth can be made from a piece of vac hose used to probe for sound. It's pretty directional, too. Insert end in ear and probe with other end, taped to a stick for the tough reaches.