Suggestions for new notification sounds?

We’ve had a few users mention they would like a different sound for desktop notifications in Mattermost.

Starting a thread for any suggestions of public domain sound effects that might be appropriate as a new notification sound.

Any suggestions?

Maybe this is a dumb idea, but couldn’t you use sounds provided by the operating system? I know the e.g. on macOS apps offer selection of various built-in sounds, like Apple’s own Mail, and (a 3rd party example) Transmission BT client.