It sounds like it ‘may’ be a fault - my own thoughts are it might perhaps be noise from the switching power supply - so I would get it checked.  Best thing to do is to try grab a video of what you’re seeing/hearing and also after reproducing the issue, immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to resolve the matter.

Hello,

I have now received the suggestion from Sonos Support to reset the Arc soundbar. This is not particularly original, but I tried it anyway. And up to now it has worked out, the humming has disappeared for the moment.

So in the end could the humming sound really be removed by resetting the soudn bar or does this not rather indicate that something is faulty with the Arc soundbar? What do you think?

Does the hum level vary when adjusting the Volume?

It is amazing the number of computer ills (Sonos speakers are at the heart computers) a simple power cycle or reset will cure. For how long is a good question though, it may be a one time glitch or a bug triggered by some condition or set of conditions.

The humming sound has disappeared for now, but I wonder that this will last for long time.