Why will my driver side window not go down?

I have a 2002 GMC Sierra SLE 1500, with half back seat. My driver side window will not go down. Before it stopped going down this window would go up and down really slow. However, now that it has stopped going up or down, it is stuck in the up position. When I push the button I can hear the motor going and I can hear some grinding or rubbing. When I do push the button downwards, the window will drop a centimeter or two and than I push the button in the up direction and the window will go up just as far as it went down. I do not believe it is the motor because I can hear it running. What else could it be? I have no doubt the motor might be going out as well but as of right now it is still running, the window is just not being lowered all the way.

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