sudden vibration

[z1-Happy2]

Can anyone tell me why my z1 vibrates every some minutes during ıt is in stand by mode (screen is off and no using).it vibrates some times evety 5 minutes some times much more longer.Each time i think there is new massage but actually just a simple vibration??? i think it was like this from the beginning not so sure.gps is off also.stamina mode is on usually.

[_alexdon_]

Do you have any app acting up(battery savers,task killers etc)?Try a force shut down(power button+vol up until the phone vibrates 3 times).Turn it on normally after.