I have seen that happen to me in 2 scenarios.
-while when machine is cutting i forget about that and cut something with the plasma cutter outside the room. I know right away that the machine misses a step, cause i have connected loudspeakers to the pc, cause when not cutting i am listening to music. So when it skips a step from inteference there is a "piu" sound in the garage

-i have one slippery, to say, collet for my 3mm bits. Some times i will not tighten it well, so the bit would dig and make something like that. but this machine is flimsy so i dont know if that could be your case