starter grinding

[adrenelin500]

a few months ago I drove my 1982 300sd on a 3 km very bumpy , deep puddle road to my cabin. The next morning I went to start my car and instead of the car starting it produced an ugly grinding noise. Tried a second time and it fired right up.For th next few days this happend quite frquently. Then for a few months I had no problems with it whatsoever. Two days ago I traveled the same road with the massive puddles once again and like before the next morning it did the same thing and continues to intermittently happen. Does anyone know if the constant heavy vibration from the road or maybe mud or excessive water that may have gotten into the starter have caused this, or if my starter is probibly going bad and this was just a coincidence?