I have the same problem with my 42" JVC (EDTV) Plasma.
It makes a kind of 'chirping' noise?
I had a repair man come over and look at mine, he said it was the power supply.
I have a two-year-old Samsung 27" SlimFit HDTV (TX-S2782H) that makes a surprisingly loud buzzing sound when turned on and the volume is muted (or when there's no av input going to the set). With the volume turned up, it's not too bad, but it's still audible.
It?s a cheap set in a spare room, but I figure it should have a few more years of usage in it. I'm using a monster surge suppresser/power filter... not much nearby for delivering interference... Any ideas?