Periodic horizontal mini patterns

Hi, any ideas what could be making the small periodic patterns (shown with red arrows --ignore the big blobs, those are just seams). That is supposed to be a 100x60mm vertical wall, aligned with y axis. (so probably can exclude PA or ghosting). PA, extrusion, PID, tuned. Using Vcore 500, 0.4 Rapido, Lgx Lite, ESUN PLA+, 210C, 85mm/s, four perimeters, outside first, 0.2 height . 4028 fan at 35% Since they are periodic I'd guess either issue with extruder gears or rail ball bearings? Maybe even overextrusion from the inside perimeters spilling over? Belt is maybe on the tighter side as well (at least compared to what other people have) if that could be a cause. If this is a common issue I'd really appreciate any advice before I start testing out things. Thanks!