How to find the source of a sound on an iPhone? In the last few weeks my iPhone occasionally makes some kind of notification sound.   It's not any of the built-in sounds, and I've gone through all the "Sounds" settings just to make sure.  It's a two-step tone on a marimba or something like it, but not the built-in one.  It happens about once every 10 minutes.

How do I figure out which app is causing the sound?   There is no other notification.  It's infrequent enough that it's hard to tell whether killing any particular app fixes it,  and I use enough apps that killing them is impractical.
Try disabling sound notification permissions for all apps that you don't need sound notifications for in your notification settings (Settings > Notifications, scroll down to Apps and disable from there)