We're building a Quadplane, and using the Octaquad (8 rotor) setup - Q_FRAME_CLASS = 3. We wanted to plug all the ESC's and servo's into the Pixhawk (get's pretty tight in there).
I think we found an issue where the analogue servos (Ele, Ail, Rud) start buzzing and the servos heat up as soon as the arming button is pressed if Q_FRAME_CLASS = 3. There is still a faint buzzing sound audible if we switch to digital servos. If we change Q_FRAME_CLASS to 0 (quad setup) the buzzing stops. We also tried earlier versions of Arduplane, and the problem is present in those versions.
We moved all the PWM outputs (servos and ESC's) to an SBUS Splitter, and the problem does not present itself there.
Let me know what logs (if any you may require).