RCA D27F750T high pitch beep noise

I have a RCA D27F750T hdtv and its about 3.5 yrs old. It makes this annoying high pitch beep noise after about a minute or two after start up and then doesnt go away for about 5 or 10 minutes later. its not coming from the speakers b/c when i turn the vol. down to zero its still there and it doesnt mess up the picture or picture quality. im guessing it might be the fly back transformer but im not sure. Is there any way that you could tell me what it might be?