Strange 'beep-beep' alarm each day at noon

since I've upgraded to 20.04 last month, I keep hearing a double beep X6 times (like an alarm clock) each day exactly at 12:00.
I didn't install any alarm programs recently and I'm pretty sure the sound comes from the OS because it is muted when I turn off the system sound.
Did anyone experience something similar? I really don't have a clue on where to start looking...

The sound is probably a sound-only notification for some event in one of your calendars. It is played by evolution data server (even if you don't use Evolution but just GNOME Calendar). The sounds can be turned off in Evolution 3.34.1 and higher (unreleased at the time of writing) in the Settings or via:

gsettings set org.gnome.evolution-data-server.calendar notify-enable-audio false

See https://unix.stackexchange.com/a/620061/501515 for further instructions and a general approach for debugging this.