I have the 55" UT50 and the Buzz is pretty distracting at times. I have an appointment for Panny to send a tech out to check it out on Wednesday. Even though I just got it, the build date is April, so I"m hoping it just needs the newer lock washer screws that Panny apparently switched in during later builds.
It's not a localized buzz from one corner or anything--pretty much equally loud from both corner vents and the center vent. So I'm hoping some boards are just loose and vibrating and all will be well when the screws are replaced/tightened. Otherwise it's clearly audible over dialogue in bright scenes and more than I can tolerate.
If not I'll likely just return it to Amazon. They're sold out of the 55" UT50, and I'm not sure I'd play the panel lottery even if not. I'd like to give the VIERA TC-L55ET5 LED set a try if the buzz isn't reduced.