Passenger Power Window Doesn't Raise or Lower

When the switch is pressed, you can hear the motor moving. The sound is like a metal 'thump thump', but the window won't raise or lower. I removed the motor and the gear on it spins it both directions when power is applied. I can manipulate the regulator manually to make the window go up and down. I haven't removed the regulator, but don't feel any broken teeth on it and the window is properly fastened to it. I have also tried re-inserting the motor with the regulator at different positions, but no luck. Is it possible the motor's internals are worn and won't work under load, but spin freely otherwise or have I missed something all together?