1. Just to rule it out… do you have MIDI cables connected? If so, try without them connected. Might be some unwanted MIDI Notes?

  2. Is the sequencer running when the sound happens?

  3. You are sure you’re not re-recording into the sliced buffer?

  4. Some FX anomaly? Switch both FX pages to None on that track.

  5. Does it still happen if you start a New Project and do the bare minimum to recreate it?