Sorry for the dark video.
Recently i build a prusa i3 with ramps and marlin. Everything looks ok, its just that whenever i want to start a print theres a weird motor noise after it did the homing. Its on the 00:5 in the video.
Any idea? 
Sounds like you’re skipping steps. Might want to turn the current up on the Z axis. Do the z couplers stop moving when that sound happens? Can’t really tell in the video.
Maybe speed is too high.
I will post another clearer video and the marlin config screenshot