I had a TV repair tech come out today to take a look at my buzzing 55" GT50. He was very knowledgeable and was happy to answer my questions regarding the buzzing, as he does have experience servicing this issue. Below is a summary of the information I was able to gather from him.
Panasonic has sent out two service bulletins regarding the buzzing on the 2012 units to their certified TV repair facilities. One of the bulletins involved a new screw kit, and those happened to be the "parts" that they hard pre-ordered for me based solely on a conversation over the phone. After removing the back panel, he checked the screws in the circuit boards. It turns out that my TV already "has the new secure screws." (Perhaps the -2 after the model number indicates units that have the new screws).
Looking at the TV from the back, there is one display circuit board on the left (a.k.a. SC board, according to the tech). On the right, there are two circuit boards, one near the top and the other toward the bottom. The upper right board (a.k.a SS board) "controls brightness" on the display. I'm not sure what the lower right circuit board is for. The power supply board is in the center of the panel.
The tech "shaved some junk" from the screw bosses that are used to mount the circuit board to the panel. All screws on all circuit boards on my unit were tight before he removed the boards to sand the screw bosses.
After re-attaching the circuit boards, we left the back panel off as we powered the TV on and played some colored slides. The buzzing was unchanged, so his efforts so far have not improved the buzzing. With the panel off, it is easy to pinpoint the buzzing coming from four coils on the circuit boards, two on the SC board and two on the SS board. The tech opined: "This buzzing is way too loud. The amount of buzzing present on dark images should be similar to what is heard on bright images. This is not normal."
The tech then called Panasonic and was told to order a new SS board (the one that controls brightness) as a replacement. I asked why not replace both boards, and the tech said that Panasonic indicated that replacing the SS board should be the only repair necessary, because it controls brightness. The replacement will hopefully be in next week, at which time the tech will try again to finally cure the buzzing with a new circuit board.
Apparently this was the third 2012 Panny PDP in my area that this tech had serviced that already had the new secure screws. In the first case, the TV owner decided to forego the replacement circuit board and simply returned his TV to Best Buy. The tech was unsure of what happened with the second case, as he was not involved in replacing the circuit board.
I'm going to give it a try with the replacement circuit board. If the new board does not quiet the buzzing, then I will be returning the TV to Amazon and looking for a different TV. The tech stated "my service team has been hearing more and more of the same comments from other plasma owners with this issue."