I have an OM-22 direct vented heating unit in which I only use kerosine. It has starting making an intermittent high pitched wine noise, which to me sounds like it is louder at the exhaust end of the unit. It appears to be more prevelant on colder days. I had one person suggest it is being caused by carbon build up on the exhaust fan, throwing the balance out of wack. I asked a service pro about it, and he says it's easier to just replace the motor and fans for $350-400. If it's just carbon, why can't I just clean the fan up myself? I can't seem to find any info on repairing one of the units online. I would love to get my hands on a repair manual, or at least get some idea of how hard it is to remove this exhaust fan and clean it up. Any thoughts would be apprieciated. Thanks.