I have a weirdness on my stepper motors I want to reason out.

On my Z axis, which is a ACME screw, I'm getting slippage or stalling. Originally I thought it was the coupler not being tight. Not so, I can look at the back end of the stepper and see the shaft actually stalls. When this is happening, I hear a grinding noise. I can physically hold the acme screw with my fingers and stall the screw.

If I restrain my X or Y stepper (they happen to be timing belt driven) I get a substantially different noise.

What I'm wondering is if the stepper shaft is manufactured separately from the armature and I'm hearing the armature spin around the shaft. Is this a possibility? These are Lin Engineering 5718M steppers. I can't swap the steppers around as the wiring is soldered in place.

All three steppers are the same make/model and I'm using the exact same setting in EMC2 excepting the steps/in is different in the Z.

Any thoughts?