Z Axis Humming in Home

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

First, your z homing feedrate is higher than z max.

#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0}  // set the homing speeds (mm/min)
#define DEFAULT_MAX_FEEDRATE          {500, 500, 3, 25}    // (mm/sec)

But also, the effective Z speed can be lower than max, if the sender program (pronterface) has a setting like Z speed 120mm/min, then manual moves will be at that speed. You need to test and find good values for Z speed.

Thanks heaps NoobMan,
I knew it had to be a simple setting change, I just couldnt see it.
Worked a treat