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

Apollo USB external power supply uses an energy saving feature that reduces power consumption when Apollo USB is not connected to the power supply, or when connected but Apollo USB 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 Apollo USB. This is normal behavior for this type of power supply, so if you notice this high pitched noise when your Apollo USB is powered off or disconnected, you can rest assured that your power supply is functioning normally.