[nns]

That sounds like electrical interference.

I would take a look at any electrical objects around the TV. Try isolating the TV in the room, no lights on, no other electronics on. An instructor of mine once found out that the cause of interference on a piece of equipment was the florescent lighting in the room.

FWIW, when I watch videos on the LCD from the PC, I usually have to crank up the volume - when nothing's playing, I hear the hissing noise too. I suspect that's what you're hearing as well - though under different circumstances.

edit, You should also consider exchanging the LCD from where you bought it. Could be defective.