It's possible for the vibrator to get stuck after a shock, like when you drop your phone.

Chances are that it can be unjammed. I had the same thing happen with my non-Nokia phone, took it to a phone repair shop, took them a day (probably because they were busy), and payed a minimal amount for their time.

In that case you won't even need a new part, and since you do hear the motor working, I'm pretty sure that part is still good.