Quote Originally Posted by iras View Post
The repetition is 14/cm (calculated as 0.71mm/ridge) My X and Y axises are both belt driven.
This leads me to believe that it is the stepper motion. For example, in order to move 50 mm, maybe it is moving in units of 0.71mm - perhaps Marlin is telling the stepper to move 0.71mm over and over again to achieve the 50mm.
This might not be drive belt related. It might just be the extruder motor doing steps and you are seeing it the print. You could over extrude by a small amount and see what happens. I would expect the frequency of the ripples to increase, but for them to be harder to see because they will 'smear' together if this is the case.