Blower Motor is slow

[LopeMX]

My G12 Series (Lennox ?) gas furnace heater blower motor turns very slow when the heat comes on (it also humms when turning). I suspect it's the blower housing (squirel cage) that may be bad. I had no problem when my air conditioner was running using the same blower. I REPLACED the starter capacitor on the motor, but it did not help my problem. I have thought that it may be the motor or the motor relay, but I'm just GUESSING. PLEASE HELP