I am experiencing something very odd that I have not seen before. I live in NC . I am getting a black dust buildup on my pleated air filter. I put a new one in and a day later I had the same thing. The air handler is in a closet in the laundry room and has a filter grille in the closet door. The house is kept clean and I can't imagine what we would be bringing in that would cause it. I have replaced filters with black dust in industrial air moving equipment, but this seems strange to me. Has anyone experienced something like this?