The start up and shut down will be the times when soot is most likely to be created. If you go through the shut down process of letting it cool slowly for 15 minutes prior to going into shut down mode where the fan blows to burn up the remaining pellets, it should eliminate some of that. Watch your next cooks, specially something like chicken or turkey skin, they definitely show the most, to see if the soot is occurring during the cooking. If it is not, I would guess most of that is from start up or shut down. I did, like you, have an issue of soot during cooking at one point. After sealing around the fan housing and replacing the controller it went away.