It's possible it could be the transfer cleaning unit, but it's more likely to be the fuser or the transfer belt itself. If the hot roller is old or dirty or the press roller isn't providing enough tension, all of the toner might not be getting fused to the paper.
If only one color were showing mottling, it would probably be the developer, but it's unlikely that all four developers went bad at the same time.
Answers & Comments
It's possible it could be the transfer cleaning unit, but it's more likely to be the fuser or the transfer belt itself. If the hot roller is old or dirty or the press roller isn't providing enough tension, all of the toner might not be getting fused to the paper.
If only one color were showing mottling, it would probably be the developer, but it's unlikely that all four developers went bad at the same time.