DIAGNOSIS Yeah, this one's common. That's coil whine from the power supply components. It's more noticeable in dark scenes because the TV is drawing less power overall, which can shift the frequency of the noise into a more audible range for some people.
FIX First, make sure your TV's firmware is fully updated. LG has released updates in the past that tweak power management to reduce this noise. Go to Settings > All Settings > Support > Software Update. If it's up to date and the noise is still bothersome, you can try enabling the TV's built-in screen shift and logo dimming features (in the OLED Care settings). This slightly varies the power load and can sometimes change the pitch.
IF THAT DIDN'T WORK If the buzz is loud enough to hear over normal viewing from your couch, it might be a defective power supply unit. Contact LG support. They're generally aware of this issue on some units and may authorize a replacement if the noise is excessive.
HEADS UP Don't try to open the TV yourself to "fix" this. The high-voltage power supply in an OLED can hold a dangerous charge long after it's unplugged. Also, the whine tends to lessen a bit as the TV ages and the components settle in.