DIAGNOSIS That's coil whine, a high-frequency vibration from the electrical components, usually the power supply board. It's more noticeable in quiet rooms and can vary with the brightness of the screen.
FIX First, check if it's coming from an external source. Unplug everything from the TV—HDMI cables, USB devices, antenna. Just leave the power cord. If the buzzing stops, plug things back in one by one to find the culprit. Sometimes a poorly shielded cable can induce noise. If the buzz is still there with everything unplugged, try the hard reset from the first question to rule out a software-induced power issue.
IF THAT DIDN'T WORK The buzz might be related to your home's power. Plug the TV into a different outlet, preferably on a different circuit, using a high-quality surge protector. Avoid using extension cords. This can sometimes eliminate ground loop or dirty power issues that the TV's power supply is amplifying.
HEADS UP If the buzz is loud, changes pitch with the picture, and persists through all these steps, it's likely a hardware fault in the power supply. This should be covered under warranty. Note when it happens (e.g., only on bright HDR content) for the service technician.