Found the reason, it was one of the roller wheels.
While disassembling and reassembling the machine, I didn’t really see anything wrong. The pulley wheels and belts looked good, and just a precaution, I switched the pulley from X-axis to Y-axis. But then I noticed something when assembling the laser carriage.
Even though the pulley and belt were not connected, I felt something odd when putting the top rail between the wheels. A little bump roughly every 4cm, and the only thing making contact was the wheels. So, I checked things again. The wheels just happen to have a roughly 4cm circumference, and wouldn’t you know it, one had a defect.
