If you tightened the observation door nut too tight(spring compressed, too), that could be another cause for the 'big bang theory'. Some folks just don't understand that some free play is designed into the system, and most AHS units are not a problem. The Fugi controller is easily adjusted to lower grate temps, but don't get toooooooo damned smart and think you can re-invent the wheel. Many smart folks have been down this road long before you and I; appreciate it, and back off on the panic situation. If you have issues of shaking like a fairy at a weenie roast, then re-thing the whole coal situation. Personally, I hope you become very adept at running that fine unit, and the comfort you and your family enjoys will make this small issue something to laugh about later.
I did the first 'Big Bank' in 1972 with two AA260's, and I learned to respect off-gasses from whatever source. Heck I visited a friend that was spraying Lacquer Paint with a hand fed stove roaring away.......that had my knees knocking and heading for any opening in the building before it became a test explosion. Yup, it scares you until you understand why it happens...reaching the lower explosive limit from off gasses equals a bad noise and projectiles flying if not secured in place......