It could be from the imaging unit, but try to clean the laser unit windows with a dry cloth. For this you must take out the imaging unit.
I have a CP305d, both drivers (PCL6 and PS) installed, the PostScript is set as default. The printer is fed from the router, and is used by a Win7 x64 HP box and a Win10 laptop. I do not normally use the PCL6 driver as it seems very complex with large bits that I do not understand.
All my pages now print the left side very faded, graduating to fully exposed on the right. I cannot see any matching fault in the help system. At 8 years old, I suppose I could simply junk the beast, but this is the only fault I can find, and it is otherwise quite reliable.