Does the sound continue if you remove the input or change sources?

yes even after unplugging the HDMI connected to the apple tv

I also tried to remove the power cord and leave the arc with no power supply overnight.

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

yes agree. just spoke with them and after running system diagnostic was identified that the sound was due to Alexa’s interference the voice service. I removed Alexa from Sonos system and well there was no alarming sound :) 

I was able to add Alexa again after the issue was resolved.

No more alarming sound!! 

Thank you guys for all your inputs!