I have a rumble in my furnace (2017 RP-178). It sounds like an Australian didgeridoo. Louder softer, louder softer.
I've noticed several things:
1. Before the burner lights (furnace fan running), there is no rumble. It only starts once the burner lights.
2. Sound is more noticeable in colder weather.
3. Sound is very noticeable from the outside furnace vent.
4. Hand over outside furnace vent makes the sound worse.
5. Sound is associated with a vibration that can be felt inside the Rpod.
6. Stove has a flame that jumps more than expected (not a very consistent flame).
I have had the propane pressure measured multiple times, and had the propane regulator replaced and recalibrated. The sound continues. I think someone else must have this problem?