Which notification sound is that, and why can't I find the setting?

The sound may be set by an app rather than your phone’s main sound settings, so identify the sender first.

On Android: check Settings > Notifications > Notification history, if available. Enable it now if it’s off—it generally won’t show earlier alerts. When the sound happens again, check the history, or long-press the notification and open its settings. You can often silence just that notification category.

On iPhone: open Notification Centre after the sound, then check Settings > Notifications > the app > Sounds. Some apps let you choose a tone inside the app; others only offer an on/off switch.

If no notification appears, check whether it happens when charging, connecting Bluetooth, or using one particular app. Those sounds may not be notification alerts at all.

If you still can’t trace it, share your phone model and whether it’s Android or iPhone so the setting can be narrowed down.