Still no noise (I've got newish ones to compare with).
I imagine the teeth probably are worn, however I can't see any effect on print quality.
This morning I ran a test print on all printers (the newest one only has 2000 hours), I can't see obvious differences in print quality that suggest that any of them need new belts. Certainly nothing like the picture earlier in this thread.
I will look out for signs that the belts need changing, (I might change them at 40000 hrs whether they need it or not!)