Does it happen when you put it on the glass or only thru the document feeder? if only when you use the doc feeder then it is something on the slit glass the small piece of glass about an inch wide next to the big sheet of glass. some times ink from a pen od dirt gets baked on the slit glass. it can be hard to get off but if you look closly then you can see it and clean it off.

You need to clean the glass with alcohol or a good cleaner. There is toner or something on the glass where the line is occurring.