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My gas furnace makes a humming noise when on Auto Heat, like as if the blower motor or capacitor was bad.

However, the blower turns on and sounds fine when thermostat is set to fan ON rather than Auto, so this indicates that the blower seems ok.

It seems like the furnace is getting the heat request, and the furnace is requesting that the blower turns on, because it is making the humming noise.

If the pilot light was out I wouldn't expect the blower to hum. Could it be that the the capacitor is bad; ould it be that the motor is not starting under heat request, but starting when set to ON, because this is a different motor speed?

There was some noise previously before the blower would start, and then shortly afterward the blower would stop. Like as if there where a 2nd rotating component having trouble.

Any ideas on what the problem is? Any help would be appreciated.

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