EE was in the process of changing oil in a tram reducer on a miner in #4 Unit. He had been checking the top frequently and hadn't noticed anything loose. When bending down beside the miner to turn the oil off, he heard a pop & a rock measuring 3'x2' & 0-4"" thick striking his hard hat & sliding down the back. He got up & walking about 4 crosscuts & found help.