Hi,
This issue has been mentioned in other forums and posts by users who found it was one of the following
- loose spindle motor mounting
- loose grubscrews on a flexible coupling which is often found between the Z stepper motor shaft and the Z ballscrew, especially on Chinese CNC routers
- mismatch or looseness on the shank of the cutter in the collet. I found this with what proved to be a 6 mm shank in a 1/4" ER11 collet. Due to the cutting angles, the cutting forces can push or pull a given cutter in or out.
- collapsed thrust bearing on the spindle motor shaft (that would be rare)
Good luck