I've had a similar problem. It turned out to be my heat exchangers were loose or not seated down and there was an exhaust leak!! While the car was running i pushed up hard on each heat exchanger and the left one was leaking as i could tell by sound. Tightened it and the problem was solved, until it came loose again. Also they say that if your throttle switch is not adjusted right(20 even clicks) it will "pop" out the tail pipe, similar to a backfire. The throttle switch might not be a really bad backfire though. Of course John could be right too with the valves. I'd check the heat exchangers and hope to get lucky
Alex