Why is my iPhone making a mystery tritone sound when the sound is on, and a mystery vibrate when set to silent? Yes, yes, I already read this answer: Why does my iOS device beep at noon every day?. I learned the same thing at the Genius Bar. But I've already turned "Sounds" off in every app that doesn't appear in my notification center. So, now what?
It turns out that there is one app that does not have a "Sounds" setting, but still produces sounds and vibrations: Calendar. It's easy to overlook the all-important setting "Calendar Alerts." Tap that, tap vibrate, scroll all the way to the bottom, and set vibration to "None." That should solve it.
Here's your problem right here:
Just turn vibration off: