d-man, Your problem may have been already solved by slim and Scott but don't get upset if it starts unexpectedly again.I had that problem on my new Pro Series 200 too but with a slightly different cause.In my case,the two slots in the bottom of the fire box where the hot gasses are blown up under the wood in the firebox would get plugged.The reason the slots would get plugged is the fact that I seldom burn round logs.My wood man is a tree surgeon and at least 95% of the logs he brings me must be split because they are so big.Most trees he takes down are old and huge.I have pictures of a 46" diameter Pecan tree that I had to split.What happens with such fire wood is it makes a perfect cover for the two gas slots and stops the flow of air from the fan.To avoid this problem,I use my garden hoe to first loosen all the ashes in the fire box.Then I pull 1/2 of the ashes to the front of the fire box and push the other 1/2 to the rear.I make a "bridge" of fire wood over the slots and then pile wood to the top of the firebox.By the time the "bridge"burns the fire is so hot it burns the slot plugs as fast as they occur and they can't plug the slots.-------Above where I said "ashes," hot coals is a more accurate description.-------Almost forgot to tell you this important tip.Take a 1/4" mild steel rod two feet long and bend it at a 90 degree angle so that you have 12" of rod on each side of the bend.Connect this to a 4 foot long 3/4" diameter pipe and you have a nice slot cleaner.I call it my Dingle Dangle.If you have a clogged slot and cover it with firewood it won't be long before you see your boiler temp. dropping and realize that you have got to pull all that hot wood out and clean the slots.That is real "outdoors sport".---------Hope someone benefits from this.A flapping clapper is an ominous sound when you don't know what is happening and why.----------shorman