This applies to the Liquid Preamps

Due to the nature of the Liquid technology, the gain control needs to be stepped. You may notice that if you operate the gain control when no signal is present then audible clicks are heard at the output of the unit. This normal behaviour for the unit, but don't worry, these clicks will not be present when there is a signal present at the input.

A detailed explanation of this can be found in this article:

Liquid Preamp Gain Clicks Explained.