I work heavily in component level electronic maintenance and repair of electronic systems such as high power radio transmitters, radar, telecommunications, computer networks, display systems, you name it.
I'm also heavily involved on a hobbyist level with silent PC design.
(Sorry for the bio, but it helps to be taken seriously)
One thing I can tell you is solid state electronics can sing and buzz. Capacitors, coils, regulators, transformers, all vibrate especially at high voltages. Power supplies, ballasts, amplifiers, exciters etc are often the culprits.
Not all components are equal, sometimes changing a capacitor with the an apparent exact copy from the same manufacturer will produce a different result. Othertimes there seems to be no way around the buzz no matter how many times you change component.
I've noticed components which seem to perform rather quietly but then find one that deviates from the norm and noticably buzz. Other stuff might just all buzz in a certain application, while paying more for something of higher quality (or simply rated for higher voltages), and there's no noticable buzz.
Sometimes the buzzing can be a serious problem for users. People can be sensitive to high frequencies at low volumes and this varries WIDELY by person.
It's important to remember that the components that are creating the buzz are not all equal.
I can say with all certaintity that Plasma TVs do not all buzz at the same levels, and frequencies. Consistency across a product line is very rare.
Some may buzz lower than most people can hear, but still be incredibly annoying to those few who can hear it, I bet a good portion of plasmas fall in this category.
Some may buzz lower than ANYONE can notice, and I don't doubt this is the case for many plasmas.
Some may buzz so bad that everbody is annoyed by it, also quite likely to happen to atleast a small portion of plasmas.
Some may be too quite for audio test gear, but that is unlikely, a truly silent high power device is very rare. (Fluorescent ballasts are going to make some noise).
A trick is to track your ear around energized components with a tube (from a paper towel roll perhaps), narrow down the culprit(s) and replace them. (Audio test equipment could accomplish a better job.)
I don't recomend that to anyone but trained proffessionals. Besides, I'm sure manufacturers don't make it easy on anyone but their own technicians to take apart and make component replacements on a plasma.
Another big thing is the structure of the TV, it's frame, the screws and bolts that hold it together, the mounts, the wall, the TV stand, the floor.
Anything that is physically attached to the TV in someway can amplify the vibrations of the components and create more sound.
Tightening or loosening screws and bolts (within reason) can often make a difference. Putting padding or seperation between parts can make a difference.
Another big thing is the acoustics of the room. Flat hard surfaces will reflect sound. For instance if your TV buzzes directionally out the back, put a soft surface on the wall behind it to prevent reflection.
I hope this helps.