There are two main reasons for the extruder motor to be vibrating.
Loose Motor Wire Connection
Sometimes when the motor wire is loose, or improperly / not securely connected it can result in the motor vibrating.
A motor wire is made up of 4 individual wires. When one of those 4 wires isn't making good connection, it can cause such an issue.
This issue can sometimes be traced back as far as the RJ45 cable or the main electronics board. Make sure all the connections along there are also secure.
If the extruder is clogged it can sometimes cause the motor to vibrate.
If the nozzle is indeed clogged, the motor will vibrate because it cannot overcome the force of the filament being clogged.
See this article on how to unclog your nozzle.