Diagnosis
That's coil whine from the power supply board. It's not a defect, but a common physical phenomenon in many electronics, exacerbated by high brightness/contrast loads.
How to Fix It
Reduce the electrical stress on the power supply. Go to Settings > Picture > Expert Settings. Turn down the Brightness and Contrast sliders slightly. Also, ensure Eco Solution or Ambient Light Detection is OFF, as these can cause constant power adjustments that increase noise.
If That Didn't Work
The noise might be a fan if you're mistaken about the source. This model doesn't have a fan, so persistent loud buzzing indicates a failing power supply component requiring a technician.
Heads Up
The sound is often more noticeable in a quiet room. If it's a high-pitched buzz that changes with the on-screen content, it's almost certainly harmless coil whine.