Z axis 'thunking' (sticking?)

I'm having a problem and I'm nearly at my wit's end.

When near Z=0 (within 2mm), some downward movements cause a 'thunk' sound.

I've been able to ascertain the following:

(0) It's a RepRapPro Mendel 3 Tricolour with 3 extruders installed.
(1) Using the Duet web interface, at Z=0.45 or Z=0.55, If I go Z+1 Z+1 Z+1, Z-1 Z-1 Z-1, the last Z-1 will usually result in a thunk sound.
(2) But doing Z+1 Z-1 or Z+1 Z+1 Z-1 Z-1 will *not* thunk.
(3) My father says he tried removing the threaded rods and flipping them upside-down, but the problem persists.

I'm having weird issues with my prints and I believe they're related to this. Does anyone have any idea what could be wrong?

this problem indicates that something is not parallel or cornered 90degrees. usually the problem is the shafts on the z axis. try mesure them on the top and at the bottom. The m5 has nothing to do with it. As it is happening only down at the prints it is telling you that the problem is down. So measure them to make sure everything are set up alright .

I have exactly the same issue. its driving me mad

Found the problem, the stepper motors for the Z axis are pinned out wrong, swap the two centre wires around (red and Green)

[www.youtube.com]