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Hi, how are you? Is this a split system? If you disconnected all the low voltage wires at the thermostat, and disconnected high voltage at the outdoor unit, yet contactor is still humming, then it is pulled in and still receiving 24v through the thermostat wire. Most likely there is a short in the low voltage wiring somewhere. Do you have a volt meter to take some voltage measurements? Also, can I get the exact model number of both your indoor unit and outdoor unit? Thank-you.