That's the hi-pitch sound on the headphones output when Medusa is not playing?

This is auto-calibration. The analog oscillators are physically running during it so even though their volume is turned all the way down for tuning, you can still hear them a wee bit in the headphone output (only, does not apply to Master out).

In 4.0 auto-calibration is happening much less often as the synthesizer heats up so you shouldn't be hearing this too often.