My Samsung J3 (specifically SM-J330FN) has suddenly started giving a curious double-vibration at random intervals. Since I never know when it's going to happen it's difficult to listen closely to the effect, but both sounds are very brief and of the same duration. Notification reminder is switched off and I can't find any other setting which would produce this. The phone is running Android 8.0.0. I'd be very grateful for any thoughts. Thanks.