I went through the burn chamber on my spare m21 heater and test fired it earlier. It worked ok so I shut it down. Went to fire it back up and it went into lockout after a few minutes so I reset it. Next time around I got a pretty loud sound like when an industrial electric motor on an air compressor starts up from the relay area. I am thinking that this may have been an issue before I tore down the heater but I just didn't notice it. Is it common for the fan heater to cause this or could it be a board problem? It almost seems like the initial current draw caused the noise. I have some minor experience with power supplies and have seen overcurrent shut down the system before. Any pointers? If I can find my ammeter I would like to do some further tests.