How to find out which app is sounding a bell, and how to silence it?

When I put on my headphone I can hear an app keep making a sound (a short bell, perhaps. It's hard to describe in writing) very frequently, almost like once or twice every second.

I have been going thru all app in 'Notifications' in System Preference and uncheck 'Play sound for notifications', for example:

but it does not seem to stop it.

The only way to completely silence it is by turning on 'Do not disturb' in notification centre.

By my observation it seems to be closely correlated to Mail's downloading messages activities

So where can I completely turn off the sound?

Solution 1:

It turns out the sounds came from the Mail app. It is the 'New messages sound'

By switching it to None I can get rid of the frequent bell sounds.