ok i only read the 1st page of responses but i have to add this input...
although i have only cockatiel experience it is quite well known that parrots, big and small alike, usually get loud when they're "establishing contact with their flock" known as "contact calls". they want to know who's around and where they are. it's how they communicate with each other out in the wild so naturally it's how they do it in our homes. i had a friend's mom who sadly got a sun conure purely on impulse due to its beauty and ended up getting rid of it cuz it was so loud and started making their cockatiels scream.
my tiels only screamed as a way of making contact with me but one only screamed when there was someone at the door or when my daughter, then a toddler, was crying in her room. i'm hard of hearing so the bird put 2 & 2 together and began alerting me to those things when i couldnt hear them. she was my hearing assistance birdy, haha.
anywho the biggest mistake to make with a screaming parrot is to yell at them. some do it to silence the bird and although it might work temporarily it'll eventually only reinforce & intensify the screaming once the bird realizes your "answering their calls".
although parrots do love to scream and some swear they do it to hear themselves, there are always exceptions to every rule. just because they mostly do it doesnt mean you might not get lucky and end up with one thats not so bad.