I spent some time with the rig this afternoon. Took the cover off the furnace: it's an Atwood 8531-IV-DCLP. Poked around a little, but taking it apart was pretty intimidating. I started it up, and after a short (20 sec?) howl, it ran quiet and seemingly smooth. Man, that baby pours out heat! I shut it down long enough to put the cover back on, and cleaned up from some other work. Turned it on again, and it started right up and ran quiet from the git-go.
I had the same thought as JaxDad about letting it howl, but now I'm wondering if maybe the problem has taken care of itself. Previous owner says he didn't remember any strange noise.
So even though extra heat is the last thing we need down here in southern Arizona right now, I'll start it up regularly when I go to work on other things. One thing seems clear: it this needs intervention, I think it's gonna be from a pro. This has passed my DIY threshold.
Thanks, all.
Rich
1999 Coachmen Santara Class C -- 24-foot
Ford V10