I've got a '94 Coleman with an Atwood furnace in it. Sometimes when it's firing up and or running, it sounds like a weed whacker with a bad bearing. I've never been into a furnace before and I don't know what could be causing the sound. I'd guess the blower wheel is intermittently making contact with the housing, creating the noise, but I'm not sure how to track down the noise... Does anyone have any suggestions?