Samsung TV buzz after being on awhile.

My Samsung TV model TXM2790F is about 3 years old. After the TV has been on for awhile, I hear a faint buzz coming from the back of it. It is very annoying if the volume is not turned up enough to drown it out. Adjusting or muting the volume makes no difference. When I change the TV/Video input, the noise may get louder or completely go away, depending on the input mode. If I shut the TV off for awhile, the noise will be gone when I turn it back on. But after awhile, it will come back again. What could be causing this, and is it fixable?