Is your truck automatic? If so, I would look at the flexplate to torque converter bolts. FP-TQ bolt sounds usually aren't consistent with RPM but it's possible if they've gotten really bad. Long shot, but it's easier than replacing things. Have you poked around with a screwdriver or stethoscope? That should at least help you pinpoint it a bit more precisely. I doubt a dented oil pan would cause noise by itself.
Looking for engine parts in the oil is a good idea and what I would do in your case. That sounds really, really bad (and admittedly, rod knock-ish) but eliminate all of the less bad things before jumping to the worst possible conclusion.