When I first turn on my furnace (on cold nights) the fan comes on and makes a real loud noise. After it runs for awhile it seems to be less noisy. It's almost like a loud fibration or maybe the fan itself needs replacing. I assume there are 2 fans in the furnace. One for safety to blow all gases out like I have on my boat and one for circulating through the camper, right? I've seen where the ball bearings on these fans when cold and worn out will produce this noise.