Accepted delivery of my replacement 55ST50 today from Ceva. No physical damage to the box or TV. The delivery guys were very accommodating (unlike the Pilot Shipping guys) and let me run through my checklist. I left the TV in the bottom portion of the box (kind of wish I didn't) and ran the slides and the buzzing seemed minimal and less than my first ST50.
After they left I set it up on the stand and started the slides and right away I noticed the buzzing was louder than it had been when inside the bottom part of the box. Wth! The box and packing materials must have dampened the buzzing. I moved the TV into the room I plan on running the slides and started the slides again. The buzzing seems just as loud as the first ST50 maybe slightly less.
Either I have super mutant hearing (X-Men sign me up), I am unlucky, or this is just the way it is with the 2012 panels - possibly a combination of all three - lol. In any case I don't really want to go through ordering another so I will run the slides and then use some test content to see if there are improvements or worsening in other areas, particularly vertical streaks (DSE) when the camera pans on a sold background, e.g. soccer/football fields, hockey rinks, cloudy or blue skies. Amazon was cool enough to let me hold onto the original ST50 for 30 days so I will choose the winner.
As far as the buzzing is concerned I may try placing a wall rug or something similar behind the TV to see if that helps. I also might call Panasonic and have them come and try tightening the screws on certain circuit boards to see if that helps.
The TV is a December 2012 build.