I got the exact same problem with my Tyros4. I am an electronic engineer so I detected the hum to be somewhat close to a multiple of the switched mode power supply frequency(5-10k). I found a loose capacitor inside the power supply, most probable due to a violent drop, or a little accident I had with the board(Some soldering inside there looked rather hasty and low quality work. This capacitator was a part of a filter, the high pitched noise was, of course, polluting the rest of the circuits, around 10k beeping tone. A little soldering one of the legs and now it appears dead silent !!!!
Hope this will help somebody out there ,cheers!!!!!