If the low frequency hum lasts for only 3to5 seconds and stops for 10 to 15 seconds and then repeats. First are you using a long extension cord along with the electrical cord that came with the PC? When the compressor on the air conditioner goes to start. There is a momentary drop in the voltage. If you use a light duty extension cord its like using a hose the size of a straw to put out a fire. On start up the voltage could drop as low as 60 to 80 volts. Which is not enough to start the compressor. ( this is also why you see the light dim)
Now if the voltage delivery is adequate we need to look at a few things in the air conditioner. First there is a start capacitor, you always get a certain amount of voltage drop when a high torque motor starts. A start capacitor is a storage compartment for voltage. When the motor starts it is normal for the voltage to drop to around 90 volts. This is when the cap is allowed to discharge and keep the supply near 120 volts.
Next there is a relay that provides voltage to the start windings of compressor momentarily to get the armature spinning.
Now to the most costly part, the compressor. the compressor has to sets of windings, a start and a run winding. Like its name the start windings are energized momentarily by the relay. Once the motor starts to turning the start windings are dropped and only the run windings are left energized. Then the compressor could be seized up.
Any one of these can cause the symptoms your experiencing . Oh and I forgot to mention the circulating fan motor. But my bet would be a cap or compressor.
