Does it appear only while using the DF? You can also manually feed a piece of paper through the DF while it's open, maybe the paper is catching something and pushing it through. On very rare occasions I have seen dirty optics, something on the mirrors themselves, cause lines like the typical scan lines due to a dirty scan slit glass. The biggest tell is if the line goes all the way edge to edge or if there's a deletion at the edges, if there's a deletion then it's optics.
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