I Agree with Mike. The hum sound is the compressor motor trying to start but it is unable to because the pressure in the compressor cylinder is the same as in the tank. This could be caused by a bad check valve or because the motor is just getting old and tired.
Be aware that when the motor does make that sound it is drawing an excessive amount of current and will overheat very quickly. This can lead to damaged armature windings and insulation and commutator contacts. You should turn it off immediately when it does that.