I thought I'd grab some data with tools around the house.
I have a PN50C8000. I used a Radio Shack SPL Meter mounted on a tripod, and placed 1 foot from the screen at various screen locations, with different inputs selected, and menus off/on, and various screen material.
First, I can just hear the Plasma at 1-2 feet away. At my seating positions I feel like I can sometimes hear it more noticiebly—with some tinnitus I have, it's hard to tell though. My house background noise—son and daughter not around-- is below the level of detection for the meter ~50dB.
With the Plasma on and warm, the peak sound is from dead center of the Plasma at 56dB. It falls off by 1 dB to either side at the middle pointed directly at the screen. To overcome and mask the sound to 57dB I have to turn the sound to "11" out of 100 from the TV's speakers—"10" puts it at 56dB on the meter. I can reliably hear sound/dialogue at "11" and as low as "2" from my listening positions, but I can't hear buzz with sound off at 3 feet from the plasma. There must be a number of frequencies beyond my detection that the plasma is putting out that the meter can read.
I found no difference in SPL by selecting different sources, menus, display of white/black, patterns, or BluRay on/off as noted above.
I suspect that the heightened sound that people hear from various positions are from an interference pattern either from the plasma as source reflecting off walls, or potentially 2 point sources from the plasma…maybe the peak I hear in the center is a maxima of an interference pattern. Nodes are generally right nearby, and quieter.
Sound falls off by 1/r, so the 57 dB the meter reads at 1 foot is 51dB at 1.5 feet by my meter, and below the level of my meter's detection beyond 2 feet. At 8 feet, 57 dB should fall to 38dB—a difference of 18dB. I can still hear that TV's sound for dialogue at "2"/100, but not the buzz from my sitting position. Check this website out for some great info on sound measurement. sengpieaudio.com/calculator-distance.htm
I don't know whether my Plasma is "normal" or not. First one I've owned. It doesn't bother me—except maybe late at night—but not sure if that's my ears buzzing or the TV. I'm young, my hearing is overall good. My kids aren't bothered by it, but notice it more than I—only when I ask about it. If someone else has an SPL meter for a "Buzzer," I'd sure be interested in what they come up with. I wouldn't hesitate to buy this set again.
I hope this helps someone.