On Saturday, June 25, 2016 at 9:17:53 AM UTC-7, Emran M. wrote:
On Sat, 25 Jun 2016 16:11:09 +0100, N_Cook wrote:

But the paper scans down in a vertical direction, which
means the rollers must be vertical.

Yet, the streaks are horizontal (in the opposite direction
in which the paper physically moves down through the scanner).

A scratch on the toner cartridge drum would make a vertical
mark, but the edges of the aperture can also make a horizontal
defect on the drum; it might be something simple (a dust line,
removable by lifting it off with a piece of sticky tape) or it might
be a defect caused by overheating and fusing of the toner.