If you've had this motor OUT of it's box or out of the plastic bag it came in and on your bench it is possible that something small, say a washer or almost anything has been sucked into the magnets of the motor by the magnetic field of the motor. STOP turning it by hand.
Disassemble the motor over a clean area of your workspace and inspect it carefully. The object inside my be hidden between the magnets and may not be noticeable at first.
I would not "run" the motor as is without a disassemble and inspection. Might be something in there that was picked up from the factory too. Doesn't matter - except to you as you need to clear the motor of what ever it is that is causing the problem BEFORE it does any damage.