This sound happens all the time at my apartment. I’ll be darned if I know what it is. Is it a bird? Is it something mechanical? Is it an alien? I think if I never find out what it is, I may go mad.
EDIT: A little more about the sound. That’s as long as the sound is; it does that, and then there will be a pause of indeterminate length, and then it will do it again. It’s relatively loud, and it’s outside. I’m not sure if there’s any pattern as to when it happens – in particular, I haven’t noticed whether or not it happens more at night, but it *does* also happen in the daytime.
EDIT 2: I just figured it out; it’s my neighbor’s parrot. I heard the noise going again, so I went out and walked around to see if I could figure out where it was coming from. I saw my neighbor’s sliding door open and his parrot chillin’ on a perch right by the door and it made the noise just as I walked by.
Spencer ex San Diego 
I’d say it sounds like alien rodents. Giant 8 legged marsupial rodents, with fangs, and armoured tentacles. They’re mostly harmless unless they find out you are a good source of organic hemoglobin.
Yep. Clearly alien rodents.
Wait, I’m not a good source of organic hemoglobin, right? … Right??