I would guess the imaging unit is the problem, take it out and have a look, you may be able to see if it has physical damage.
I have this persistent issue where wavy horizontal lines are present on all my prints. Otherwise the printing quality is satisfactory. I hadn't used the printer for many years, it was storaged properly (no dust). It's basically brand new. Also I don't remember having this issue back when I used to use this printer. I replaced all 4 toner cartridges but the problem persists.
The lines are sometimes like a raibow, other times they're black or yellow. Any ideas about what I can do to fix this? Thanks in advance, I have attached a photo (https://i.imgur.com/pz46n2Q.jpg).