What does "Feed Thru" refer to? Is the first laid on the glass and the 2nd through the Document Feeder?
Does the line occur in line with the paper feed direction or is it oposite?
It looks like an issue with the scan light causing a digital artifact, does this happen on print only or does it also happen on copies?
If it does not happen on copies I would eliminate the light as a possibility and start looking into the scan hardware/software.
So if digital only and copies are fine, how about switching from pdf to Tiff or jpg , does the line disappear or change?