Hi Simpson309!
I have the exact same problem with my 27" LED Cinema Display. Buzzing from the back and also from under the screen. First I suspected the fan but now I am more into the theory that the power supply is the problem, just like you are saying.
Is the sound like the sound in this videon?
http://www.youtube.com/watch?v=RQv0mswkj9s
In fact I think it is the capacitors on the power supply that are failing/getting old and I plan to replace them as Apple Service most definitly are either going to put a brand new (expensive) power supply in there or replace the wrong part like in your (and many others) case.
What has happened since your last post? Have you found a solution?
Cheers!