Diagnosis
That's a ground loop or electrical interference, usually coming through the connection with your TV.
How to Fix It
The most common fix is to isolate the audio connection. If you're using the HDMI-ARC connection, unplug it and switch to the included optical cable. Optical cables transmit light, not electricity, so they can't carry a ground hum. Set your TV audio output to optical and set the soundbar source to 'OPT'. The hum should disappear immediately if that was the cause.
If That Didn't Work
If you're already on optical or the hum persists, try plugging the soundbar's power adapter into a different outlet, preferably one on a different circuit than your TV and other gear. Sometimes power conditioners on the same circuit can cause noise.
Heads Up
Don't ignore a loud, sharp buzzing—that can indicate a failing component inside the soundbar. A low, steady hum is almost always a connection/power issue, not a hardware failure.