My Atwood 8535 IV furnace is making an intermittent squealing noise, blower rotation actually seems quiet, it is more like a high pitched vibration as versus rotation noise. This high pitched squeal typically occurs shortly after the blower start running, or sometimes after it has run for awhile. The longer the blower is running the more likely the squealing is to stop. This is why I have pretty much eliminated blower motor rotation as the source. I can't see anything in the fan blades and I have a mud dubber screen on the exhaust. Any ideas would be greatly appreciated before I start taking things apart.