Y belt dual peak

Hey everyone, I just converted a core XY printer to klipper and use the accelometer of a beacon Rev. H for belt tension and Input Shaper. I've fairly got some experience with building 3d printers and also with klipper, but I can't get rid of this 125Hz peak on the Y axis. The X looks absolutely dope with 14k mzv. I aligned and squared the gantry, made sure that belt tension is equal, zip tied everything that could move and checked every screw. I'd like to at least get below 1.5% vib and over 5k accel on mzv. I'm on the tighter side of belt tension with ~150hz over belt length of 150mm. I replaced the belts when upgrading to klipper, the frame should be very rigid (40mm extrusions and steel plates for xy plane). I begin to think if I should put a normal ADXL to the part of the toolhead which is mounted directly to the toolplate = to the carriage, as the beacon is obviously mounted on the downside and could therefore in theory move around more. Also because the toolhead is my own design (based on vcore toolhead setup) and thus could have some imperfections regarding stiffness.