Tiny has probably called it correctly. A wrist pin usually makes a "clucking" sound, although I suppose you could argue that it is a rear main bearing. in my (limited) experience, rear mains make kind of a "rumbling" sound at idle and just above.
If it were mine: I'd drop the pan before it got worse, and check the rod clearance on the crank. You can probably remove a shim or so if need be. Take a real close look at the oil pickup scoops and the oil pan oil carriers while you're in there. Procedure is described nicely in the shop manual for your truck.
