DIAGNOSIS This is a power surge through the subwoofer's amplifier when the signal changes state. It is a known characteristic of this model's circuitry.
FIX There is no permanent fix. You can minimize it. Always lower the volume before changing inputs or turning off the system. Use the remote's power button, not unplugging.
IF THAT DIDN'T WORK Ensure the subwoofer is on a dedicated power outlet if possible. Sharing an outlet with high-draw appliances like a fridge can introduce voltage fluctuations.
HEADS UP A single pop is normal. Continuous popping or buzzing while the system is on indicates a failing amplifier module in the subwoofer. That requires service.