Possible clue!! After removing the burner tray, inspecting the heat exchanger fins, putting it all back, and testing propane lines for leaks, etc...... I fired it up (started with a boom), but I let it run for about 20 minutes. All of a sudden I heard some buzzing from the propane valve, and then it got a bit louder, like a chatter. I verified that the furnace was still running and heating, and put my hand around the valve. I could feel some vibration. That doesn't seem right. The supply control voltage on the display shows about 28 volts, but I think I need to check it at the valve. I am going to order a new valve too.