One of the local Avian population is sitting on the chimney and telling it's compatriots all about it. The noise is being amplified down the chimney so it sounds like I've got a Crow in the stove.
Perhaps you have. A neighbour once came to ask if I could investigate a strange noise coming from her fireplace which had a screen and freestanding electric heater. I removed both and found a somewhat confused pigeon which had come down the chimney.