I cannot hear it all the time, just occasionally after few hours of work on cMP and it does ramp up and down in loudness until it disappears again. Does it sounds like a fan or HDD to you? I recorded the clip with phone when it was at its loudest point (you can hear it ramping down) and boosted gain 9dB in DAW so you can hear it as clearly as possible. Any troubleshooting advice? Thanks!