We have our coach parked right outside our bedroom ready to be loaded for our next trip. The last few days have been in the 20's so I put the furnace on to keep everything from freezing up. The problem is when the furnace comes on it makes a deep rumbling sound. The sound is loud enough to hear inside the house. We had the furnace on last season and it did not sound that loud or maybe it only sounds like that outside. Is this the way it should sound?