There is a good thread on Pradopoint about reconditioning the starter motor, which I have followed and managed to get it working again. I had same issue with my D4D 120, it would just click, and nothing would happen. This might happen several times, until it eventually it turns, not a good feeling when driving auto trans in middle of nowhere. It is due to water ingress as mentioned. If you go through deep water crossings, it will start happening eventually. I now carry a spare one with me (second hand one which I took apart and cleaned), if going for a longer trip.