Ok, I took the truck out again and I can replicate the noise.
It's definitely not the bumper shackles banging. It actually sounds like more of a high pitched "chuff."
Question: If your exhaust system is crappy, can a diesel back-fart through the exhaust the same way gasoline engines can?
These sounds are definitely not the air compressor. I know what that cycle sounds like. The sound is definitely "in front" of me and down around knee level or lower.
I recorded some footage.
The video is jumpy garbage, ignore it. Just skip to these time indexes, close your eyes and listen for the "stand out" sound:
02:45-02:51 (one "pop" at 02:50)
03:05-03:09 (a few pops)
04:54
05:17 (single, pretty audible pop or bang or slap)
6:50-6:55 (several)
My phone's microphone doesn't seem very sensitive to high pitched metallic sounds. I can hear the noise on the footage but it sounds very minor in the footage compared to the engine and the gear whine.
I'm starting to agree that I'm sweating over nothing but I thought you guys should have a listen.