Dirty printing

Question

I have an HP3550 colour printer which is printing a marbled stripe of all colours on the edge of the page. Two print quality tests were fine apart from a slight mark on the yellow. I changed this toner and ran the test again and all pages had the marbled mark on both sides. It may have got worse when the printer charges itself with a new toner but I have no idea why or what part to replace. I would be grateful for some advice if possible.