Computer won't stop making a "Windows" sound, even if sound turned to mute

For background, my laptop is a Dell Latitude E6410, a few years old, and it uses Windows 7. My computer also uses Avast antivirus, the free version.

This started about an hour ago. It will consistently make the "exclamation" noise, once every minute or so, even if I have the volume on my laptop muted. I went to Control Panel > Hardware and Sound > Change System Sounds, and discovered the exact sound was called "Windows Exclamation". Turning off the Exclamation sound on my computer did not stop the noise from happening.

I also tried restarting my laptop, but that didn't help. Made the noise consistently while the computer was shutting down and while it was booting up. No prompt comes up when the noise occurs, either. I was not on any malicious or unknown websites, either (browser was Chrome, if that helps). My next option is to run a boot scan, I guess, but I was hoping to hear if someone else knows what's going on before I put my computer out of commission for a couple hours.