DIAGNOSIS The scanner carriage is stuck or its belt has jumped off the track. The carriage hits the end of its range and knocks.
FIX Unplug the printer. Gently lift the scanner lid. Look at the white scanner carriage under the glass on the right side. Manually, and very gently, try to slide it toward the center of the printer. If it moves, guide it to the center. Check the white plastic belt above it for any twists or disengagement. Ensure it's seated on the pulleys. Plug the printer back in. It should re-home.
IF THAT DIDN'T WORK The scanner unit's position sensor is faulty. The unit doesn't know where it is. This requires a full scanner assembly replacement.
HEADS UP Never force the scanner carriage. If it doesn't move smoothly with light pressure, stop. Forcing it can break the stepper motor shaft.