My android email client apparently pushed some sort of update in the middle of the night that changed how and when it decides to make semi-quiet little "bingly-bingly-boop" noises, and I went from "my MUA makes no noise, ever" to getting 1-15 of those in any given minute.
It took a couple of hours to figure out what the fuck was doing it, because notification history claimed no new notifications were being issued.
And this was on the same device I use to lull myself to sleep with podcasts.