DIAGNOSIS Coil whine from the power supply unit. It's a high-frequency vibration from inductors or transformers under load.
FIX Go to Settings > Picture > Advanced Settings. Turn off 'Local Dimming' and 'Contrast Enhancement'. If the noise stops, it's power supply whine under high brightness load. You can try reducing peak brightness in HDR settings.
IF THAT DIDN'T WORK The noise could be from the cooling fan. Ensure the vents on the back are not blocked. The fan is not user-serviceable.
HEADS UP This is often a hardware defect, not a software one. If it's loud and persistent from day one, contact Hisense support for a warranty replacement. It won't get better.