Hi terraplane, thanks for writing. I have experienced this once before, and in my instance, despite my apparent thoroughness to fully separate the output speaker cables, because the binding posts are really close together, I found that any clicking was indeed Sprout going into protection mode (the clicking is the output relay) and that this protection mode was being caused by the speaker cables/connectors shorting on the output. It may not be a permanent solution, but as a test, can you fit a slip of paper or a business card between said banana plugs and ensure this separation? Once completed, if still clicking, there may be an internal issue with that channel. Sprout is enjoying a <.5% failure rate worldwide, and I have not seen this problem before, but I suppose anything is possible. Would you mind trying my experiment?
Of course if the speaker cables are shorting anywhere else, such as the loudspeaker side, the same issue will occur, just to keep your eye out.
Please feel free to call my direct line if I can offer further clarification or of course should we need to make you an RMA. 720-402-3941.