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.