You ever had a machine doing a job in the back ground and suddenly the sound changes from what you expect?
Was just running an engraving job on the laser and suddenly there was a click and the sound suddenly got much quieter. Freaked me out. Jumped up and looked at the laser and it was doing it’s thing fine, exhaust was running and then I realized my air bush pump, I use for air assist, had cut off. I had forgotten to open the shut off valve back up after changing out some wood and the pump had hit it’s pressure limit cutoff. 
It can really get the heart a hammering 