My AC mains 60 Hz (however, good question!)
Other than irritating (and not really all that bad, seeing as the cooling fan is about the same total dBa), it was more of a concern that it might be indicating something (else) wrong with the unit.
I could see running one of these at 400 Hz on an aircraft, but the aircraft probably couldn't get off the ground with this thing aboard.
Thanks for your description/info, much appreciated!
Dave
