This happens because of a loose component holding the vibration motor. While tapping the phone does work, for me, it was only a temporary solution. The issue inevitably got more serious, and I’d end up beating the phone against my hand (not even tapping, at this point) every 1-2 minutes of using the phone. It was impossible to continue using the phone this way, so I took it to a third party service center (because authorized service centers in India suck and they refused to fix the issue) and had the issue fixed in 10 min and it cost me close to nothing.
It’s an easy fix, all you need is to tighten the loose component/screw, so I suggest taking the phone in to a service center or repair shop to fix the issue for good.