Macbook Playing a Notification Chime with No Apparent Cause

I've just updated to Catalina, and I've noticed that my MacBook will often play a chime periodically throughout it's usage, and I can't find the cause of this sound.

I hesitate to say a word other than chime because it's not one of the sound effects found under system preferences (like Basso), but very distinctive and I've heard it before. I believe that it's "finished downloading" sound, or an updated sound.

But I've checked and I have automatic updates off, no app is updating, I'm downloading nothing, and as far as I can tell the chime isn't originating from my browser.

The best way to describe the chime is three mid-range notes with slight reverb, the last note higher than the others.

Please help in deciphering this phenomenon!