If you're wondering just how loud a sun conure is, I uploaded this video of Stewie "vocalizing":
http://www.youtube.com/watch?v=bYFMo5Ar1k0
He can get louder than this, but it's pretty representative of what his usual screaming sounds like.
Obviously he doesn't scream 24/7, but if you're thinking of getting a bird and you have no idea what an aratinga conure sounds like, you might want to know what you're getting yourself into when you fall in love with a sun conure.