Samsung A7 auto vibration issue

[Android Central Question]

My Samsung A7 is going automatic vibration mode from ringtone mode several times without doing nothing. I have tried to find out any setting who could do this, but I can't find any setting like this. So, I have hard reset my phone to solve this problem but not solved yet. Even I didn't set any app who is responsible for this. Please help to stop automatic vibration mode.