Does it happen on prints and copies? Probably a drum.
But if only on copies I would then assume it is a straight line and only happens when using the document feeder, and if that is the case I would further assume that it is a solid line that goes oerfectly straight across the page in the direction the paper is fed. And again, if that assumption is correct, lift the document feeder and look at the narrow strip of glass, probably on the left side, I believe you will find something on the glass, typically a solid (whiteout). If so just scrape with the side of a bank card/ ID card and wipe with a rag (Don't use windex)
Obviously I have made assumptions and this may not be helpful.
This pic has the piece of glass circled I referenced
