Diagnosis
This is electrical noise, either from a ground loop or interference on the power line. It's a noise from the internal electronics, not the audio drivers.
How to Fix It
Isolate the soundbar's power source. This is the most effective first step.
1. Unplug every audio cable (HDMI, optical) from the soundbar, leaving only the power cord.
2. Turn the soundbar on. If the buzzing is gone, the noise is being introduced through a connected device.
3. Plug cables back in one at a time, starting with the optical cable (if used). Optical is immune to ground loops.
4. If the buzz returns with HDMI, you have a ground loop. Plug the TV and the soundbar into the same power strip or wall outlet.
If That Didn't Work
The internal power supply might be defective or overly sensitive. Try a different power outlet in another room on a different circuit. If the buzz disappears there, the original outlet has 'dirty' power.
Heads Up
Do NOT use a cheap 'ground loop isolator' on the HDMI cable—it can break the CEC and ARC functions. If you must isolate a persistent HDMI ground loop, use an optical audio connection instead, which physically breaks the electrical connection.