If you've ever noticed a high pitched mechanical whine being emitted from your Apollo Twin's power supply when the Apollo Twin is powered off or disconnected, here's why:

Apollo Twin's external power supply uses an energy saving feature that reduces power consumption when Apollo Twin is not connected to the power supply, or when connected but Apollo Twin is powered off. A side effect of this energy saving feature is that the power supply will emit a high pitched noise when it is not supplying power to the Twin. This is normal behavior for this type of power supply, so if you notice this high pitched noise when your Twin is powered off or disconnected, you can rest assured that your power supply is functioning normally.