Chimney Swifts
Chimney swifts are small migratory federally protected birds about the size of a Robin, that nest in chimneys from April-October, once gone we can install a cap to prevent them from returning next year, if you remove them we will not return for any work.
Here a video was sent to us by a Customer of the noice they make.
https://en.wikipedia.org/wiki/Chimney_swift