If you mean the outside condenser is humming although you don't have the thermostat set on call for cool (when was the last time you did have it on cool?) than it could be a combination stuck contactor (allowing power to get to the capacitor) and/or (if a single leg contactor and one leg of 120 volts is constant) simply a bad capacitor.
What kind of condenser is it? Is it a heat pump? Does it work when you turn it on?
"Heat pumps" use the outside unit for heating. Is it a heat pump?
And they are making these very efficient lately. If it is just humming but the fan is not going, that could be a normal "minimal heat needed" operating mode. Write down the model numbers and call the manufacturer or look at their web site for an owner's manual - which should explain various modes of operation.
If it's a heat pump, and it has one of the old mechanical thermostats, it's normal that sometimes these thermostats apply power to the reversing valve even though no heat or cooling is being called for.
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