I wouldn't be too concerned. If the stove is working properly and there are no sulfer smell or gas alarms going off, then I would say everything is OK. One thing to always look for is sweat on the bottom side of the hopper lid. If you see condensation on the lid, you can bet your going to or are having a draft problem. This happens when your draft slows down to a point where the fumes are not being sucked out of the stove. The stove becomes pressurized and the fumes are looking for a way out. Since the stove is pretty tight, the easies way is up through the hopper. This makes the coal sweat, vaporize and then condensate on the lid when it cannot excape. Have a gas alarm on every floor of your house. If any alarm goes, turn the stove off ASAP.