Hi Anthony,
That clicking noise is the relays inside the DAC, which work to fully mute the output of the DAC when there are changes to the DSP chain that would otherwise cause audible clicks or pops to come through the headphones. Things like sample rate changes in the content you are playing are a good example of this.
So basically, the click is when the DAC performs a ‘hard mute’ where a ‘soft mute’ (done in the software of the unit) is not enough to stop pops coming through the headphones.
Nothing to be worried about, that’s just the DAC operating as it is supposed to.