For a few months now we've been finding dead or near death flies in the house. About one every two weeks. Another dead fly on the floor. They looked like regular house flies. Wife had our fireplace all decorated and we haven't had a fire all winter. It's a gas fireplace. She put away the winter decorations and is transition to spring but it gave me a chance to clean some soot from the bricks that have been there for two years. Upon opening the fireplace doors, I've discovered the source of the flies. These flies look a little greener than regular black house flies. I assume there is or was some food source in the chimney for them. I did get a slight whiff of death a few weeks ago but don't smell it much now. Do I need to call a chimney sweep to clean out whatever might be in there or can I wait and see what happens? Any additional information is helpful so I'm not totally naive if the sweep tries telling what is needed.