never actually seen a U2 in real life, but any parrot that screams didnt just "suddenly" start screaming.
my suspicion is that it's either an attention thing or an adjusting to new surrounding things. 6 months ago i adopted a cockatoo which was found and taken to the local avian vet. screams like a nazgul -no really he does. 6 months on he's better but still carries on. the hardest thing is to figure out whats upsetting him. with Freddy, it's either an attention thing or he has a different idea to his routine to what i do.
for example he'd scream non-stop in the evenings in his cage (he cant come out because he attacks my missus, and early on i didnt let him have free reign because he used to attack me). So i eventually had to put him into a night cage in another room. this calmed him down. my suspicion is he was used to going to bed at a certain time and liked to be somewhere quiet. So now i know at a certain time BEFORE he starts carrying on I move him and it's usually a lot quieter.
if you had sunshine in a cage near you would he keep screaming? I tried putting freddy in a cage and having him near me at night but he kept screaming. just thinking whilst he settles in he could be in a smaller cage but still be around you to know he's safe.
other idea i keep hearing is to make the cage enjoyable so they're happy being in the cage. I've spent literally thousands on toys for my birds but i still find they enjoy the non-parrot toys best lol
i've spent heaps on indiivdual toys *I* think thye'll enjoy but then find they like chewing on the cardboard box i gave them
my red tail black used to whine incessantly as well, but i eventually figured out she only did it when i was around and she could see me. when she couldnt see me she was fine and would just forage and do whatever, but if she knew i was nearby she'd carry on.
i know theoretically we shouldnt be isolating birds for noise, but i've found with these 2 it actually settles them, something to consider trying