Hello happy that you have found a solution partially to your problem. As for the main problem I did describe at the beginning of this post, yours does not look to be exactly similar, but as you said it is caused by outside devices plugged to your Monitor.
The problem I did describe is not related to anything plugged to the monitor. The hissing sound is perceptible whatever if you have any external devices or not plugged into the TB Display (other than the Thunderbolt wire from the TBD to your laptop, letting it on battery power or not).
I'm not sure if your problem is related or not, but my problem was that is a very perceptible static hissing sound that you can hear while any audio is playing, it also continues for 30 seconds when the audio/video has stop. You can usually reproduce the hissing sound by simply selecting a media in the Finder (like a quicktime movie) and let Finder initiates the Preview in the last column. You can also initiate the problem by simply opening the application FaceTime or iChat. You will hear the static noise for 30 seconds unless you begin using the speakers from those application (then the hissing would be cover by the sound). When I'm talking about a perceptible sound, for sure I mean in quiet environment. This is why most of the poeple did not notice this problem I think, probably using the TBD in less quiet environment.
I never found any solution to my problem, ever after having replace my TBD 4 times (yes 4 times) or having plugged it in a friend house with another laptop. The problem was persisting on each TBD.
I finally did decide to buy a refurbished iMac for $450 more than the display and cannot be more pleased. i synch everything between my MBA and the Mac using iCloud, iTunes Match and Dropbox. This is why I did stop participating here, I lost hope about Apple beign able to fix the problem, at least for this version of the TBD.
