DIAGNOSIS That's coil whine—a high-frequency vibration from internal power components, usually the power supply board.
FIX Check if the buzz changes with screen brightness. Go to Menu > Picture > Backlight and lower the setting from 100 to 80 or 90. If the buzz diminishes, the backlight power circuits are the source, and lowering the setting is your fix.
IF THAT DIDN'T WORK Ensure the TV is on a stable, level surface and the back panel isn't touching anything. Sometimes vibration transfers to a media console. Try placing a soft, dense material (like a felt pad) between the TV stand and the unit it sits on.
HEADS UP Coil whine is generally harmless but can be annoying. If it's loud and constant from day one, it might be a sign of a defective power board—contact Vizio support while under warranty.