Yep, the first nationwide Emergency Alert Squeal test. Wait, here it comes!
That was weird. All I heard was this.
Update: That, of course, was from a list of Star Trek sounds. My fave (not that I listened to them all) is this one, undoubtedly from one of the movies after they figured out that Bones' "Dammit Jim, I'm a doctor not a _______" had comic possibilities.