I own a Blaze King propane fireplace insert. I vacuumed the fan last summer for the first time since we owned it (6 years ago when we bought the house). It was just filled with dust. It ran quietly before that & up to this winter, but has started to whistle terribly. It doesn't do it all the time when the fan is running. Comes & goes. What could be the problem & how can I fix it.