Dell 5100CN grey background

My printer has 40,035 total images to date. In the span of a couple of pages it went from perfect print, to a light grey background on each page. The printer was not hot at the time.  Printed 3 pages fine.  Printed them again fine a few minutes later. Third time page 2 starts the pattern. Page three is as dark as it gets, and all subsequent pages have the same level of darkness to them.  Again, even at its darkest, it is what I would call a light grey.

The grey background is paper edge to paper edge (outside the printable area). It is a fairly fine dusting, that looks random when examined closely, yet there seems to be some slight vertical structure. As you look across the page left to right, some vertical zones are very slightly darker or lighter than others.  There is also some horizontal structure, but it is far less noticeable.

The intended part of the printing is fully black.  There is no smearing or smudging of either the intended print, or the grey area.

This must not be common, I don't see others describing similar symptoms, except for a pink background.  There is no color at all in the background I see.

My printer is long out of warrantee, so I'll most likely be order parts and fixing this myself, if I had a clue what this might be.

Do you know what causes this?