My pixi 4 keeps buzzing for no reason, how do I turn it off?

My daughter's pixi 4 keeps buzzing and vibrating randomly - when not moving or being notified of anything - and it does so constantly night and day. Switching off notification noises does not stop it from continually and randomly buzzing and vibrating. Please can someone advise us how to switch the buzzing / vibrating off and be able to use the phone normally?