We have a similar problem with the furnace in our "new" 2004 Tahoe Lite. Took it out for our first camping trip, everything worked, includeing the Suburban furnace, but the fan is loud and sometimes rattles like crazy. It's not a squeak.
I assume there is something loose inside so I'm going to take the whole unit out to check it. (Just not sure if I'll screw up the propane system when I turn off the propane and disconnect the gas line, can I just reconnect it and turn the propane back on and be ok)?
I think I also need to remount it on a rubber pad.
I've had other TT's with Suburban furnaces and they have been nice and quiet running. This one ain't quiet. Does this sound typical for a 2004 furnace?