It's not necessarily common, but excessive buzzing is the warning sign and/or end result of various component failures inside the TV. The most common cause is a bad/vibrating coil on one of the various PCB boards inside the TV or a failing power supply, but sometimes it's a failing panel (which is rare though). Usually a simple board replacement or power supply replacement fixes the problem. My friend's first 60ST30 buzzed when it was delivered so we put it right back in the box and sent it back with the driver. His replacement the following weekend was silent at one foot away and remained silent for several months, then one day it suddenly developed a loud buzz that could be heard from 10 feet away, coming from the board in the upper left corner of the TV. The Panny technician did a little troubleshooting then replaced the board and the TV has been silent ever since (about two years now).
If he tries to tell you that it's normal to hear a buzz beyond a few feet then he's full of it - insist they fix it or replace it.