Some Tonic drone presets can make a small noise when changing from preset to preset. This is mentioned in the Sunday Keys Resource Page videos about Tonic as well, and some workarounds are mentioned. You will just want to turn Tonic off, and then you can fade in the next preset Manually, or via the Automator on the next patch. This is unfortunately a MainStage limitation and not something we can address with the way MIDI works in MainStage.