Hi @engrmusaqasim
It could be a loose or faulty vibrate motor or perhaps a problem on the systemboard.
Try replacing the vibrate motor and check if that resolves the problem.
Here's an image taken from this teardown [https://www.youtube.com/watch?v=K-lZoXnyMKg|video] that shows where it is located in the phone.
[image|3042967]
(click on image)
Search online for ''GH31-00788A'' to find suppliers of the part.
If it is still no good after replacing the motor here's the troubleshooting flowchart from the service manual and also zoomed in images and normal size images showing the location of the components mentioned in the flowchart. ''Note'': ANT5000 and ANT5001are the motor flex cable board connection terminals
[image|3042968]
[image|3042970]
[image|3042969]
(click on images)