I would guess that a dedicated bus for the Metronome is necessary to more easily control the volume of the 'click'?
I was going to suggest opening the Metronome settings under Preferences and adjust the volume to 'zero' to solve the problem you're experiencing, though as it's project specific, you'd have to do this every time.
There's also the toggle buttons on the Control Bar in the metronome section that allow (or not) the 'click' to appear on recording or playback.