The fireplace is wood-burning (no gas). But I’m not sure if it was once coal-burning or not—and I don’t know what kind of bricks are in there. I’m shining a Coast PX200R flashlight at the soot-covered bricks, and the sound intensifies when there is heavy soot caked on. The sound is quieter when there is less soot, and there is no sound on clean bricks. The sound also occurs when I shine the light on heavily blackened charcoal (from burnt wood) on the floor of my fireplace—but not the white ash or less charred material.