DIAGNOSIS This is electrical interference, often ground loop noise. It is introduced via the HDMI or power connection.
FIX Isolate the source. Disconnect all cables from the soundbar except the power cord. Power it on. If the buzz is gone, reconnect cables one by one. The culprit is usually the HDMI cable connecting to the TV.
IF THAT DIDN'T WORK Plug the soundbar and the TV into the same power strip or wall outlet. This equalizes the ground potential. A different HDMI cable may also resolve it.
HEADS UP Do not use a ground loop isolator on the HDMI connection. If the noise persists with only power connected, the soundbar's internal amplifier may be faulty.