Window doesn't stop going down?

I have a 2002 Astro Van. When is lower the driver side window using the auto down feature the window motor never stops. The window goes all the way down but the motor continues to pull on the window and makes a loud clicking sound. When I push the on again as if I wanted to have the window go up then the motor stops. It is as if the window motor doesn't know the window is already down. What causes this and what is the fix? Thanks!!!