The arm and cartridges slam into the sides of the unit or move in fast jerking motions back and forth.
The encoder belt may have slipped out of its track in the back of the head carriage unit or it may have become unseated from the posts on either side of the unit.
The print head carriage has a sensor behind it that reads the encoder belt to help orient the head carriage so it knows where to stop left and right, where to pick and where to start printing.
The encoder belt is the clear plastic strip about 1/4" in width inside the unit running parallel to the metal bar that the arm and cartridges glide on. (Not the white ribbon cable connected to the print carriage.) It looks tinted because there are many little vertical lines on it that the head carriage reads like a road map to find its way left and right. If the encoder belt becomes dirty or slips out of the track on the back of the head carriage then the head carriage becomes blind and loses its way. The result is the arm not knowing where to stop, slamming into the side of the unit or stalling out completely with the left/ink LED flickering rapidly.
To fix this, reseat the belt on the metal posts on either side of the unit if necessary. Also, make sure the belt is inside the plastic notch on the back of the print head carriage. Finally, clean the belt using rubbing alcohol and a soft cloth. Pinch the belt with the cloth between your thumb and fore finger to clean both the front and back of the belt.