Normally, Ozzie (my alex) is not really loud, he will only scream in the morning and late afternoon, and it only last for about 5 minutes. Unless he is very existed, he will sceam a bit. But in general, my cockatiels are actually noisier than Ozzie, but Ozzie is louder when he screams.
I personally don't think Ozzie is really loud, but my parents sometimes does. I think it relate on how loud you can handle. I can handle cockatoo screams, probably because I have lots of wild ones around my house, so I'm already used to the sound. If I compare Ozzie with the cockatoos, Ozzie is nothing! I usually get hundreds of cockatoos flying around my house screaming before sun set.