R62 fire unexplained>

Raining here today so getting a break to catch up and take a breath. Last Saturday, I had a wedding to go to so my son was cutting beans without any one around. I trust him more than any 25 year old I know, he is very mechanically inclined. He cut around the field, turned around to head back and smelled smoke in the tail wind. He found a 3 foot fire right at the driver side rwa drive motor (where all the chaff rests). Had an extinguisher mounted a foot from the rear entry ladder, he put it out. Burned the covering off the rwa hyd lines under shoe above rear axle. But nothing else looked hurt. We thought maybe the rwa motor or solenoid got to hot or sparked. Next morning we found burn spots a couple of feet in diameter in the area he cut for a long ways. So we decided to leave rwa off. But had no idea how it started, that's a sick feeling. That was Saturday at 5pm we cut beans all day Sunday and Monday until 3pm. He was unloading in the cart that I was in when he came out of the combine in a big hurry. Said he smelled it again. Engine off at this point. He started looking in the area of the rwa, hoses, solenoid, so on. I opened the engine compartment and found puffs of smoke coming from the inside of the C channel of the rear engine main frame directly on the floor. The driver side is capped and have no access. one 1/2 hole under bell housing, smoking. The right side has partial access but smoke was coming out from under the c channel the length of the oil pan from a very small opening. Got some water and compressor to dowse the best we could but very little access. As we were blowing the channel out we noticed smoke coming from the ground next to left rear wheel. An ember had gotten blown out of the c channel, fell to the ground and started burning. Now we know where it came from but not what started it. We cleaned and blew water the best we could. Cut beans Tuesday, Wednesday, then switched to corn Thursday and Friday with no hint of smoke.