Ideas for diagnosing random terrible noise?

[Rollo Grande]

Ahhh these suggestions are setting up a good project plan, thank you! What a helpful forum.

A mechanic buddy is letting me use his shop, so I come over in the evenings and flounder around, and if I get stuck he bails me out. Pulling in last night he yanked back and forth on the bed and said that it might be loose and shifting on the frame. I don't think that's the source, but it did look a bit loose compared to the cab. He did that also while I was under the truck with a flashlight, I couldn't see anything wrong, but maybe not looking at the right supports.

I'll be solo for the next few weeks while he's on vacation so limited to noob wrenching activities and probably a lot of cleanup and prep. I am good at "paint and shutters"....