Diagnosis
That's coil whine from the power supply circuitry. It's a common physical trait in many electronics, not a defect, but it can be amplified by certain brightness/color settings.
How to Fix It
Go into the Picture menu and change the BrilliantColor setting. Toggle it between Off, 1, 2, 3, 4, 5, and 10. One of these levels will significantly reduce or eliminate the noise. Usually, mid-levels like 3 or 4 are the quietest.
If That Didn't Work
Enable the Eco or SmartEco lamp mode. This lowers the lamp power, which often reduces the electrical load and the associated whine.
Heads Up
The noise is most noticeable in a very quiet room and from close range. From your seating position with audio playing, it's often inaudible.