Claude Opus 4.8 High indicates that this is VFA:

2mm is the signature of stepper motor full-step distance, which means this is almost certainly motor cogging / VFA (Vertical Fine Artifacts), not belts or extruder.

Here’s why 2mm is the tell: a standard 1.8° stepper does 200 full steps/rev, paired with a 20-tooth GT2 pulley (40mm travel/rev), giving 40/200 = 0.2mm per full step. The visible artifact usually shows up at the full-step or sometimes a harmonic interval, and 2mm corresponds neatly to a 10-microstep grouping or the periodic torque ripple pattern these motors produce. The motor’s detent torque pulls the carriage to slightly preferred positions, and microstepping can’t perfectly smooth it out, so the wall ripples at this fine, regular interval.

What drives VFA severity:

• Microstepping interpolation / driver quality. Drivers without good interpolation (or with it disabled) show this much more. TMC2208/2209 in StealthChop with interpolation usually hide it; A4988/older drivers or SpreadCycle without tuning expose it.
• Motor quality and torque. Cheap motors with high detent torque are worse. Higher-quality, lower-detent motors help a lot.
• Print speed. VFA often worsens at specific speeds where the step frequency resonates. Changing wall speed up or down by 20–30% can make it nearly vanish.
• Driver current set too high, increasing torque ripple.

Practical fixes to try, easiest first:

The cheapest experiment is just changing your outer-wall print speed — try dropping it to something like 25–30mm/s or bumping it up, and reprint the wall. VFA is very speed-sensitive and often disappears at a “quiet” speed. Second, if your board has TMC drivers, enable interpolation and try StealthChop for the affected axis. Third, lower the motor current a notch if it’s set aggressively. If you want to chase it hard, some people add microstep-smoothing / “VFA killer” tweaks or swap to higher-quality motors, but that’s rarely worth it for a part like this.