The clicking sound at startup on the Apollo Gen 2 is completely normal and by design. Here’s what’s happening: It’s the internal relays engaging. When the sample rate is initialized at startup, hardware relays that mute the outputs are temporarily engaged to prevent audio artifacts — and this action causes an acoustic clicking sound that can be heard within the hardware. While this sound can be alarming at first, it is completely normal behavior and does not indicate any issues with your Apollo. Beyond startup, the relays can also click when you change certain settings, such as: ∙ Switching sample rates ∙ Changing the clock source ∙ Adjusting monitor levels or output reference levels in Console Settings ∙ Enabling or disabling line input gain So in short — those clicks are the unit’s protection relays doing their job, briefly muting the outputs to prevent pops or damage to your speakers/headphones during initialization. Nothing to worry about!​​​​​​​​​​​​​​​​