Antenna interference

You could be experiencing a ground loop problem. Try plugging the Sub into a different outlet or power strip.

Thank you for your answer. Did that, still no joy

If you move the Sub to a different room, do you still hear the noise? If so, you should try resetting the Sub by following the steps in this article and redoing the setup process:

https://support.sonos.com/s/article/1096?language=en_US
 

If resetting doesn’t work, I would contact Sonos Support:

https://support.sonos.com/s/contact

Also, if you do perhaps have the option, maybe see if the noise you’re experiencing from the Sub goes away when you unbond it from the ARC and re-bond it with a different Sonos device/room.

If the noise goes away, then you may want to consider power-cycling the ARC aswell, before trying to bond it back.

I’ll give it a try. Thank you

Does the level of this sound track the Volume control? Does the sound change if you reorient SUB slightly?

Sound does not change. It is not a constant sound. It is just as if it is picking up a radio wave

And, in the Sonos app, I do not get the option to change the sub channel, even if I hardwire the sub to the router

Other than WiFi devices, are you aware of any nearby radio transmitters?

If this is some sort of radio interference, physically moving SUB to another location will at least change the character of this interference. SUB uses a private 5GHz channel when Bonded with ARC. You have no control over this channel.

Likely there is a  hardware issue with SUB and you’ll need to work with SONOS support. 

Meanwhile, try using a high quality power line surge suppressor and keep SUB’s power cord bundled as short as possible. Older TV’s often included a “Ferrite Bead” and users were instructed to pass the TV’s power cord through this device. If you have one of these, clamp it over SUB’s power cord, close to the SUB end.

You may want to try pulling the power lead out of the Sub, rotating it 180 degrees, plug it back in and see if that helps with the noise.