It’s possible you’re running at a speed that makes the machinery resonate. Changing the speed 20% up or down might eliminate it (if it’s resonance).
I’m not certain this is relevant, but there is a Chordal motion associated with the belt tooth lifting off the belt pulleys. Russ Sadler found a cure for it by changing to a belt that is better at damping vibration and by reversing the belt so the chordal motion can’t interfere with the position of the engrave head.
It shows up very clearly as stripes or bands when engraving images.
Full analysis: RDWorks Learning Lab 83 Is this Curtains for ever - YouTube
Solution: RDWorks Learning Lab 121 Is this CURTAINS for CURTAINS - YouTube