I have a three year old Arcoaire RPJII, model NTC5100BFE/GNJ100N12D. The first season it performed flawlessly. At the end of the second season it began making a terible metal grating noise every time it would start the heat cycle. The noise sometimes subsides after the unit gets hot, but it is loud enough to wake a person from a sound sleep. I have isolated the problem to thecombustion blower assembly unit. my question Must I replace the whole blower unit or can adjustments be made to the fan blades or cage to eliminate the scraping ?