I played with it in jog mode and found 2 spots along the z axis that make this snapping sound.

they only make the sound from a dead stop....it's fairly hard to find in jog mode causing me to believe the spot is very small. One locations is much louder then the other, and they are 100% repeatable (if you hit the jog at the right spot)

i was was thingking a stepper gear, but that doesn't make sense because the stepper would rotate dozens of times over the z axis travel.....then the snap should happen in dozens of places, but so far only in 2 spots