I have a Day Night model 100cu-12b furnace.I just bought this house and the furnace looks to be from the 70's era but this furnace is new to me. For it's age, it's in reasonably good condition. It works properly except for one thing. When the burners light there is a very loud booming noise,sometimes three rapid booms in a row. It has a standing pilot that is clean and positioned properly. The burner flame color and pattern look perfect. The air mix doors on the burners are showing soot on the outside indicating to me that flame is coming through them when the event occurs. I had a "Pro" look at it and he could not find anything wrong. He said it was old and if I wanted to fix it I should replace the furnace. As I said I just bought this house, putting another two to three grand into it is just not in the cards right now. Does anybody have any suggestions? I have a few ideas of my own such as slow opening gas valve vs fast opening gas valve, possibly a retrofit pilot system that will light all the burners at once instead of hopping from burner to burner (3 booms). One thing I find questionable is that when the pilot flame is out it can be lit without pushing a pilot button or knob. Maybe this has something to do with it?