The rattle could either be something loose inside the furnace or the fan motor bearings are about ready to seize up.
On my last trailer mine started that way then the fan quit turning.
I was able to purchase a used identical Hydro-Flame furnace from Colaw's RV salvage for about the price of another fan motor for my old one. When it arrived it had never been used, the heat exchanger was still silver, indicating never been hot. Do ask when ordering that they send the connector with it, as the newer one's are pinned different and they can be a pain to reconfigure, also ask for a roll of the butyl tape sealant as well.
It took about an hour to change out, then another hour to figure out that I should have asked for the connectors for the thermostat, I managed to push the pins out with a kabob skewer and reconfigure to get it to work.