Very happy to see, that I'm not alone. Very few poeple are hearing this sound to my understanding and it is difficult to know if they simply do not hear it or do not experience the problem at all.
I did change the display 3 times already, and even have replaced the Thunderbolt port in my MacBook Air 13 inches and the problem is still persisting. I must say that the hissing sound is more suttle since the replacement of the port, but I wonder if I have just been used to it by now.
I think this is a general problem experience by all, it is just that some poeple noticing it and some don't.
But clearly it is a sound that you can hear from 2-3 feet away in a silent room.
What you are experiencing in your post is EXACTLY, to the letter, what I'm experiencing.
To get a fix, we need to problem to be recognized by the engineers of Apple. Something that would not be easy if we are only 2-3 persons complaining.
I did elevate my problem to the engineer level, but didn't hear anything from them yet. If you call them, please let me know your experience.
In the mean time we need to decide if we keep the display or not as the problem doesn't look to be with our MacBook Air/Pro unless it is realy the Thunderbolt port that let pass some static sound by the wire.
I didn't say in my earlier post, but I did try with a TV with a HDMI wire plugged to the Thunderbolt display via a adaptor, and didn't hear any hissing sound coming from the TV speakers. It may not be entirely the same connections used as with the Thunderbolt display, but it is another test I did.