My Lopi sounds like a swarm of angry bees are up in the stack when it gets a good secondary burn. I know there are not any bees up there, lol, but seriously-is it the draft? Is that normal?? It's only been doing it recently, so maybe it's the draft combined with the wind we've been getting lately-but it's disconcerting to someone that doesn't really care for bees, especially after last summer's experiance with them at our old house (we had a nest of yellow jackets in the wall, which ended up in the spare room-1000's of them, at once).