Same thing JUST happened to my 55 inch VT30. I have had it since July 2011. Just today a very loud buzzing could be heard. Sitting 10 feet away, the buzzing was very obvious. I knew something was wrong.
My warranty is up (ended less than one month ago), so I came across your post and decided to go at the TV with a screwdriver. I took the back panel off, and tightened all the screws on the boards as well as any other screw I could spot. One of the screws on a board on the left hand side of the TV (when looking from the back) was quite loose, otherwise the rest of the screws only took about one more turn by hand.
I did also notice this!!!:
Missing screw and rubber bumper on one of the fans!! It was not anywhere in the case of the TV. It must have never been installed. On a good note, it doesn't seem to be the cause of the noise. But if anyone happens to know how I can find a replacement screw and bumper, I would appreciate it because I feel like to should fix it. This is what is missing:
Anyway, thanks for the inspiration. TV is back to normal. Only the normal, baseline buzzing can be heard now when you put your head real close to the TV. Also the knowledge that my TV is lacking a screw!