Followup.
Since doing a carbon rod cleaning, I have not experienced this noise at all.
Since the process involves loosening and retightening the tensioning screws at the back of the unit, this both suggests that it is something to do with the belts, and fiddling with the tensioning screws relieves the issue.
Of course, I have a sample space of one; the above may just be a coincidence rather than causitive. 