Hi,
I have exactly the same problem with my display - a disctinct buzzing sound that is easily audible even in an office environment (while I concur that the buzzing is at it's highest @ brightness level 50, the audible range would vary from 35-75).
My display has already been sent to the service once. It had panel issues - unit arrived (even on the very first day I could hear this sound), worked for a day or two, started showing vertical and horizontal stripes on the right-half side of the display, and then wouldn't turn on anymore. Took 2 weeks, display came back from the service yesterday, while the panel has been switched with a working one, I can still hear the buzzing.
I am using a retina MacBook Pro (Late 2013), have tested it with Dell XPS 13 (2013 early? model)
What's up with that, even with "3-Years Advanced Exchange Service and Premium Panel Guarantee" Dell fails to deliver a problem free unit?
I need help. :(
Thanks,
Mario