I have an iPhone 13 Pro running iOS 15.6, which I got in January. Every now and then it will do a very specific, distinct pattern of four vibration pulses that lasts around two seconds, and which feels like it's the hardware trying to notify me of something specific.
There are no indicators on the screen and no other notifications that go out (either on the phone or on my Apple Watch). Often it happens when I physically sit down in a chair, but sometimes it happens when I've just been sitting. It doesn't seem to have any correlation to activity from other devices nearby. Because there's no other sound I have no idea if it happens without the phone being in my pocket.
I have tried holding the buttons in all possible combinations and none of them cause this specific vibration pattern to occur; the "emergency/SOS" vibration is the closest thing but its vibration pattern is totally different (being a couple of brief clicks), and is accompanied by other effects (such as FaceID shutting off and a bunch of UX for contacting emergency services popping up).
It's always exactly four pulses (bzzz bzzz bzzz bzzz), and it seems like the sort of thing that would be a specific diagnostic code from the phone.
I have contacted Apple support about this and they have no idea.
It generally happens once or twice a day, at totally random times. The phone is always locked and when I unlock it, it goes straight to the application launcher/home screen. It seems unlikely to be a notification coming from an app, and I've disabled push notifications on pretty much every app anyway.
Does anyone have any idea what's going on?