DIAGNOSIS Dust or a hair is on the underside of the scanner glass or on the white reference strip (the narrow strip along the top edge inside the lid). The scanner carriage moves under the glass.
FIX Lift the document feeder. Unscrew the two screws at the top rear of the scanner glass assembly. Carefully lift the glass panel. Clean the underside of the glass and the white plastic strip beneath it with a glass cleaner and microfiber cloth. Reassemble.
IF THAT DIDN'T WORK The scanner carriage itself might be dirty. Manually move it to the center and inspect the long, narrow scan bar (the CIS contact image sensor) for debris. Clean it gently with a dry swab.
HEADS UP Unplug the printer before cleaning inside the scanner area. The carriage can move unexpectedly if power is on.