There was a pair of barn owls that nested every year in the barn on the property I lived at. The first year I was there, one night when I walked into the barn, once their chicks had grown large enough to peer over the nest, they let out a very loud blood curdling hiss/rattle that sounded a lot like a rattlesnakes. Scared the heck out of me until I figured out where the sound was coming from. Every morning and every evening when I would go into the barn the parents would take off and swoop very close to me letting out that very eerie screech.