Diagnosis
That's coil whine. It's a high-frequency buzz or hum caused by electrical components (inductors or capacitors) on the main power board vibrating as current passes through them. It's common and often harmless, but annoying.
How to Fix It
First, try to isolate the cause. Turn off all features like 'Ambient Mode' and 'Auto Brightness' in the settings, as these can cause the power draw to fluctuate and induce whine. The most effective fix is often to change the TV's 'Energy Saving' mode. Go to Settings > General & Privacy > Power and Energy Saving > Brightness Optimization. Try switching between 'Off', 'Low', and 'Medium'. Changing this alters the power delivery and can sometimes shift the electrical frequency enough to stop the buzzing.
If That Didn't Work
If the buzz changes pitch with bright scenes or is very loud, it could be a slightly faulty power supply component. Since it's a safety issue to open the TV, your best bet here is to contact Samsung support. They are aware of coil whine in some units and may offer a main board replacement under warranty if the noise is deemed excessive.
Heads Up
Don't confuse this with the normal fan sound. Some TVs have a small cooling fan that spins up when hot—that's a lower, blowing sound. Coil whine is a steady, electronic buzz or squeal. It's often more noticeable in a quiet room when the volume is low.