I appreciate all the responses, folks!
I’ve ordered a noise reducing bonnet; the ear plug idea is second in line if I don’t see an improvement. Trailer camera is on standby, I’m still looking at options.
I had my XC lesson yesterday and I made SO drive me back while I was in the trailer with my gelding. Previously SO has only driven me with the trailer empty for safety reasons. He didn’t love the idea but eventually relented.
There are a few things rattling with a loaded horse that weren’t obvious empty. One was a big spare cotter pin up by the divider above my gelding’s head. I’ve taped that down with some electrical tape. There’s some creaking/wind gusts from the windows above the ramp - they are secured down, but slam against the ramp over bumps. I’m going to ask my trailer guy to come out next weekend to fix that. If anyone has any ideas how to get that battened down, I’d love to hear them.
The chestbar joints creak with horse pressure on them - I WD40’d the joints and then lined it with vet wrap.
The ride itself was smoother than I expected. Much less bumpy and jarring than empty, even over potholes,
Observing his behavior in the trailer, my gelding seemed calm once the trailer started moving. His breathing settled, and he wasn’t splay legged or struggling to keep balanced - another reason I wanted to sit back there and see what was going on.
I did not try hoof boots yet - I pulled mine out of winter storage and found that a rodent of some sort had made a midden in the boot box and destroyed one of my hoof boots - so that idea is on standby until I can order some more.
I’m not sure these are the root of the issue, but I think addressing them will help. I’ll keep you all updated and keep any other ideas you have coming!