Xperia Z3 compact keeps vibrating non stop!!

I bought a refurbished Z3 compact (worse decision I ever made). The first time I turned on the phone, it started vibrating instantly and kept on vibrating until the battery died. I flashed marshmallow ROM hoping that it would solve the vibrating issue but no luck. If I shake the phone, the vibration stops but after a few seconds it starts vibrating again.

I replaced the Flex cable with a new one that I bought from aliexpress but issue still persists. While I was replacing the Flex cable, I noticed small salt deposits on the flex cable connector on the motherboard. Another thing I noticed is that if I touch the spinner part of the vibrator with my finger, it starts spinning and if I touch it slightly again, it stops vibrating.

I thinking about buying a new motherboard but before I do that I have to be absolutely 100% sure that vibration is caused by a fault in the motherboard.

Any Suggestions?