Prusa i3 AutoBedLevelling and Servo Vibration

Hi all,
i add today to my i3 (Ramps 1.4) ABL support but I have 2 issue:

1) when i start printing, i add a G28 and a G29 for ABL and all is ok. The problem is that the printer doesn't go down until Z0 but start to print in the space
2) the ABL servo sometimes vibrate and move a little bit. Is any parameter that i can check to resolve it?

This is the settings in the marlin after doing the ABL calibration

// these are the offsets to the probe relative to the extruder tip (Hotend - Probe)
  //X:8.00 Y:10.00 Z:39.80
  #define X_PROBE_OFFSET_FROM_EXTRUDER -8
  #define Y_PROBE_OFFSET_FROM_EXTRUDER -10
  #define Z_PROBE_OFFSET_FROM_EXTRUDER -39.80

  #define Z_RAISE_BEFORE_HOMING 4       // (in mm) Raise Z before homing (G28) for Probe Clearance.
                                        // Be sure you have this distance over your Z_MAX_POS in case

  #define XY_TRAVEL_SPEED 8000         // X and Y axis travel speed between probes, in mm/min

  #define Z_RAISE_BEFORE_PROBING 10    //How much the extruder will be raised before traveling to the first probing point.
  #define Z_RAISE_BETWEEN_PROBINGS 3  //How much the extruder will be raised when traveling from between next probing points

  //#define Z_PROBE_SLED // turn on if you have a z-probe mounted on a sled like those designed by Charles Bell
  #define SLED_DOCKING_OFFSET 0 // the extra distance the X axis must travel to pickup the sled. 0 should be fine but you can push it further if you'd like.

Thanks

There is a section in the firmware to help with the servo moving. I cant remember the line exactly but I think its something like disable servo? by default its 300 i believe and it worked for me.

Look for this line in configuration.h
// #define PROBE_SERVO_DEACTIVATION_DELAY 300

remove the '//' lines before it to enable it