My Central units fan just hums most of the time. Only set on heat. If I have the cooling on, it runs fine. But if I turn on the heat, it just hums, sounds stuck. Most of the time I can get it going by switching from AUTO to ON (for the FAN), can someone please help:confused:

You have a multispeed motor. Sounds like the winding for the lower speed tap is defective. Pull the motor and take to a motor repair shop to get a accurate determination of condition.