One more thing, even when no cables are connected to the unit, the amp remains hot. As a precaution, I have turned off the unit

Just to let you know the normal operating temperature for the Sonos Amp is between 32° to 104° F (0 to 40° C) - so if it’s within those levels then there is perhaps less concern. It will be warm to the touch. 

The hum could be a power supply issue, have you tried it connected to a different power socket?

What do you have actually connected to the Amp during normal operation?

I would certainly start by submitting a system diagnostic with the Amp playing normally and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can see from that report and maybe confirm the unit is working correctly and perhaps offer some suggestions to resolve your issue, depending on your actual setup.

Does it hum with no cables other than the speakers connected?

If it does can you flip the power cable and see if that changes?

Virtually any unit connected to AC power will hum and this is normal. Unfortunately, it’s not easy for us to remotely decide if the hum level is unreasonable. As my screen name suggests (“buzz”, get it) I tend to be unreasonably sensitive to these things (I can be a pain in the neck) and AMP does not annoy me.

Is AMP physically touching anything that might mechanically amplify the hum? Does the hum level change based on AMP’s physical orientation? Hum tends to originate from a point and when your ear is closer to that point the hum will be louder,  but there should not be any gross changes in level when reorienting the unit. You could gently bump or shake the unit. This should not change the hum level.

Can you characterize the pitch of this hum? “Hum” is at power line frequency. There is another potential source of noise that will be at a much higher frequency. Some people will label this as “hum” and confuse us.

Hello,

Thank you all for your help!

the Sonos Amp unit has a hum as soon as I plug in the power cable. It starts very lightly at first and doesn't change if I plug in something else, like the speakers.

The speakers themselves don't produce any hum.

The noise becomes louder when I play music. The hum is the same whether I flip the power cable or change the power socket.

The sound is similar to what you can hear in this video, but it's lighter: https://www.youtube.com/watch?v=BRBu5W3fGAY"

It’s probably the switching power supply in the Amp. I would take a video of what you’re hearing and submit a system diagnostic and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can suggest to resolve the issue.

If you want to eliminate a very unlikely and very esoteric potential cause of the mechanical hum that you are experiencing, don’t connect AMP to power through a UPS (Uninterruptable Power Supply). Another very rare possibility is that your house power has an unusual harmonic content. If this is the case I think that you would have other items making some noise. This sort of noise might change by time of day or be sensitive to the number of appliances operating. Without using instruments, the only way to check for this would be to take AMP to a different neighborhood and try it out. If there is a large industrial site nearby, its influence on the harmonic content could change by time of day.

If the hum level changes over time while no speakers are connected, the hum might be related to AMP’s temperature. This is not shocking because the mechanical characteristics of certain parts change with temperature. With speakers connected and after AMP has been playing for a while, the hum level may change as AMP heats up.