Hi @AlanSMichigan
Welcome to the Sonos Community!
That sounds awful! Unfortunately, when you reboot a speaker all of the logs are cleared so the diagnostic you submitted after the reboot contains no useful information. If this happens again, please submit a support diagnostic before rebooting the speaker and while it is still making the noise (though turn the volume down first, if you can!) and then contact our support team at your convenience.
In an attempt to try to prevent this from ever happening again in the first place, however, I recommend you remove the surrounds from your Arc -Settings » System » [room with Arc] » Remove Surrounds (putting them in separate, temporary “rooms” after, if the app asks) - and then factory reset only the problem speaker. Add it back on to your system as if new, then bond both Era 300s back to your Arc - Settings » System » [room with Arc] » Add Surrounds if not prompted to by the app.
If this does not help, repeating the process but instead resetting only the Arc could fix the problem. These are steps that our technical support team will probably suggest, so performing them before getting in touch could speed up the process to the point where we just replace the speaker if nothing else works.
I hope this helps.