If it is regular with about 2 mm period then it looks like pulleys do not match belts?
But the fact that moving horizontaly from center to a side and back results in different height in the center would indicate either motors skipping steps or noise in endstops and you do not have ENDSTOPS_ONLY_FOR_HOMING defined.
Or maybe a pulley is slipping on the motor shaft?
But the fact that moving horizontaly from center to a side and back results in different height in the center would indicate either motors skipping steps or noise in endstops and you do not have ENDSTOPS_ONLY_FOR_HOMING defined.
Or maybe a pulley is slipping on the motor shaft?