The whine you hear is probably right at 400hz, one can hear it in many birds, it is the same frequency as the aircraft's AC systems. The bulk of the electrical power used on the aircraft is of this frequency, so it can be expected to bleed through from time to time. You can hear the identical noise standing next to a jetway power supply - also 400hz. One also notices that its never heard from small, piston aircraft with an exclusively DC electrical system. Lots of other squeals and scratchiness, but never the 400hz hum.