I own an Ultra sx 80 furnace and during the night we awoke to this screechy sound. Reminded me of either a dry pully. After a minute or so the heater came on and the noise stopped. Within another few minutes it went off and screeched again. When the heater came back on it stayed on and seemed to be working well. An hour later another small screech but didn't last long. What do you suppose is the problem?