If you noticed a high-pitched mechanical whine being emitted from your Apollo's power supply, here are some possible reasons why.
Some Apollo power supplies use an energy saving feature that reduces power consumption when Apollo is not connected, or when Apollo's power is switched off. A normal side effect of this feature is that the power supply will emit a high pitched noise when it is not supplying power to Apollo. The power supply can also be disconnected from the electrical outlet to eliminate the noise.
When using a Thunderbolt device with a laptop, plugging in an externally-powered device such as Apollo or Satellite can also trigger the charging system when running on battery power. This is normal behavior due to Thunderbolt specification as the ports are designed to accept any incoming charge available.
With UA Thunderbolt devices, the charge will be insufficient for charging the host computer. A high-pitched mechanical whine noise from the hardware or power supply may also be heard as a result of the added power load.
In these scenarios, the computer's original power adapter should always be plugged in when UAD-2 hardware devices are active. This will prevent the above from occurring and also regulate system power management away from the battery; both of which will promote connection stability.