Well, once again the collective wisdom of this board is spot on. Thanks everyone! It was the fan. I still couldn't see or feel the PING hit when the truck was running, but looking at the base of the radiator, there was a piece of metal placed under it to support it (the frame piece is rusted out underneath) and this looked like it had gotten chewed up a little bit. I took the fan off and sure enough, could see the bright spots at the end of the blades, where the fan was contacting that piece of metal.
All I actually had to do was smack that metal slab with a hammer to tap it back out of reach of the fan. And the TINK was gone! (Yeah I'm gonna have to bolt that thing down I guess.)
Those of you who mentioned that you hear the sound of the water pump bearing going bad, thanks for the tip! That low howling sound does indeed go away if I unhook the fan belt. I had noticed that sound before but had decided it was the sound of air intake through the cleaner! I inspected the weep hole, but it was completely crusted over with grease and S Alabama clay , so it didn't tell me much. But I'll order a replacement pump and get that replaced before it fails on me. Y'all's educated ears may have saved me a breakdown! (I have been getting most of my parts from Classic Industries. Does anyone have experience with their replacement water pump, or have a superior one to recommend?)
One final note that you guys will think is funny. Yesterday when I was scratching my head over the bad noise, I noticed that there was a little oil stain in my driveway that I hadn't noticed happen during the oil change. Now to my surprise, sitting right in the middle of it was this:
That's the thimble from a Monopoly set. I just stared at it sitting there in the oil puddle thinking, how the heck did that get there (I don't own the board game), and what if that TINK TINK noise is the sound of MONOPOLY PIECES RATTLING AROUND IN MY ENGINE?
And then I spent a minute trying to decide if the noise sounded more like the dog or the top hat.
Thanks again, all, for weighing in. I'm glad my problem turned out to be super simple, borderline stupid actually. But I thought I'd ruled out fan contact because I couldn't see or feel it happening, but your consensus to the contrary was spot on and put me back on the right track.
-Will