I could only think that a bird was caught in the chimney flu and trying to get out.
But that seemed like such a weird explanation, having lived here for 23 years and catching no bird there before.
I had to go to the porch and look at the chimney.
The problem was all on the outside – a woodpecker hammering its bill on the metal. The morning sun must have been just right to give an image of a tree trunk there to fool the bird.