Okay, 2F engine with 177k on it, starting to get a tapping that sounds like it's from the bottom end (not the valves). It doesn't seem to be noticeable when the truck is first started, but after it warms up a bit it's definitely audible. It's in synch with RPMs of the engine, just a modest tapping. This is a NEW NOISE that showed up a little while ago and has become louder. Nothing wrong with performance, not overheating, gas mileage has not changed noticeably, just changed the oil and there's no metal that I can see. Sooooooo, whaddya think it is? Looking to get it checked out *professionally* soon, but I want to be prepared.