Z Axis Humming in Home (2 replies)

Hi
I'm doing my first build and am now at the calibration stage however I'm having issues with my Z Axis when trying to home.
Ok heres all the specs.
Printer is an i3 variant (Wilson TS)
Ramps 1.4
Firmware - Ive tried several versions of Marlin and all do the the same. Currently have Marlin V1 from here
Heres a copy of my current Config file Configuration.h
Also using Repetier-Host software

All axis work fine in either direction under manual control.
X & Y both go to the home position when the home button is pressed for those axis.
I can run the z axis from top to bottom without issue under manual control (10mm at a time)
When I press the Z Home button the motors just hum.

I have calculated the pot settings for my motors which is 0.504v
Initially they were set at 0.81v, I have now got the Z pot set at 0.53v ( Ive tried from 0.44v upto around 0.7v)

any help would be appreciated
Cheers
Rod