Hello. I have a Sears 600 wall mounted furnace. This is the only information available as it is very old. Recently, when the furnace kicks on, the blower starts in a low speed as it was doing before, but after the 1st cycle the speed of the blower motor increases too much making a loud noise and keep coming on and off very 10 seconds for few seconds. There is no switch for the blower and i have changed my thermostat but still the same problem. Thank you for your help. Coomar