Still haven't nailed down exactly what's going on. The clatter noise has now stopped happening as I drive on side streets, the alley, or other low speed roads. Hot or cold, no noise. The only time I hear it now is at highway speed, & only when I accelerate(sounds alot like pre-ignition).
No idea if this next wrinkle is related or not, but if not it's quite a coincidence. I noticed this morning a wet black sooty spot on the driveway right under where my tailpipe exits the truck. Then I noticed the pipe had the same on the rim.
This lead me to check the PCV valve, which was packed solid & the hose collapsed. I replaced both this evening, so we'll see tomorrow.