Does it happen with your rMBP? Mine is a mid-2012 15" model.. the room has to be completely silent. As soon as I turn on the keyboard backlight, a static sound starts.. it sounds just like white noise. Does this happen with yours? I have this laptop since almost launch day and never noticed that.. fact is, I almost never used the keyboard backlight either so I don't know if it was always there or not.
Try to get close to the keyboard with your ear if you don't hear it at first.
Try to get close to the keyboard with your ear if you don't hear it at first.