OK, I'm done chasing this demon for now. I'm going to have to wait till something breaks to figure out what is breaking.
But after all the opening and shutting and analysis and listening and all that, I think I may go back to my initial thoughts on this issue from over a year ago, when I was first driving the truck daily.
On any given day it would run rough and loud, but it was inconsistent. In my pea-brain, I figured this must have something to do with fueling. So I experimented with different fuels and additives, and it seemed to respond a bit. Then I swapped injectors, and that really seemed to smooth it out. I even retarded the timing on the IP to get rid of the white smoke that was plaguing my neighbors. But this knock/ring noise persisted. At first, I think it was also inconsistent. But in reality, I've sort of lost track of one thing leading to the other.
All of this to say that I'm coming back to the thought that this might be some kind of pre-ignition, latent, retarded, advanced knock that is being caused by an IP pump. A few people mentioned something along these lines, so I held onto the thought, and am now coming full circle.
I reset my IP timing, and although the knock is still there, I find myself driving, stopping, driving, and later realizing that I wasn't bothered by the sound while sitting at a light! This really is new, and it tells me something about how much it has diminished.
So I think I will run it as is for a bit, monitor the knock, then see about getting the IP serviced. If that doesn't fix it, I know for sure I'm in for some kind of break down the road. But.... whatever.