Diagnosis
That’s coil whine, a high-frequency noise from the power supply components. It’s not dangerous, but it can be irritating, especially in a quiet room.
How to Fix It
First, check if it’s related to picture brightness. 1) Go to Settings > Picture > Expert Settings. 2) Turn off any auto-brightness features like ‘Eco Sensor’ or ‘Auto Brightness’. 3) Manually lower the ‘Backlight’ or ‘Brightness’ setting. Often, the whine changes pitch or stops when the power draw to the LED backlight changes.
If That Didn't Work
The issue might be specific to the internal power supply component. If the buzz is loud and constant, and you’re still within your return/exchange window, you might consider swapping the TV, as the level of coil whine can vary unit-to-unit.
Heads Up
This sound is often more noticeable during bright, white scenes (like a hockey game or a spreadsheet). If lowering the backlight fixes it, you’ve found the workaround.