have a wood insert with insulated ss liner, pro installed in masonry chimney. Right above where old fireplace damper is, the smoke chamber wall seem to begin and I can feel the bottom of it as juts out from the facade of fireplace. It feels like block that has open cores facing downward. This is the Best pic I can get of it. The cores feel to be about 1.5 inches.The open core I’m referring to is on left middle side of pic. It seems as though there are several of these going across the lenght of the masonry firebox. My thought is creosote could have built up in those cores in block when open fireplace had been used. My liner runs uninsulated near this area. (Insulation wrap starts about 16 inches up the liner from insert). The bottom of blocks itself appear clean, fwiw. Not sure if I’m worrying for nothing. Is there a need for anything to get done to increase safety here? Pic is of looking at inside front of masonry chimney, about 4 or 5 inches above damper.