According to my children our television makes a very annoying high-pitched hum - we (their parents) can never hear it though. I think it is one of those sounds that can only be heard by the under 25'sish. Like this kind of thing [broken link removed]
I'd try setting the TV to factory default settings. Then I'd try plugging it in in a different room without anything else connected to it to try eliminate the possibility that other devices or your socket are causing a 50Hz buzz problem - if the problem went away then maybe you need something like a ground-loop isolator. If the problem persists then I'd suspect a faulty power supply and I'd be returning it.