Any idea why my cockatiel has become really noisy?

could definitely have a lot to do with him not being out with you enough. its important that they are social, and get a chance to stretch their wings. i know that if i haven't had my cockatiel out for a while, he'll let me know with the shrieking. also your birdy may be a little lonely. sometimes when they're getting lonely they'll start wanting to look for a mate, in the wild they use their pretty (or not so pretty) voices to attract their mates.

good luck.

You might try spending more time out of the cage with him. Also during the day if he shreiks ignore him until he makes a plesant noise. I had to do this with my african grey. she had learned from her previous owner that to get attention she had to scream. this created a hellish enviroment for me! so now i go by the "mommy can't hear bad noises" system. that may help. only give attention for good noises.

So in the am when he starts shreiking don't yell "shut up!" because in his world, that means you are giving him attention, and hence shreiking works. Make it not work. (I hope this makes sense :-) )

Also does your bird have a very thick dark colored cage cover? I noticed that if I just throw a blanket over my grey she will start making noises as soon as the sun comes up. however if I put her navy blue cover on her that blocks out all the light she usually won't make noise until I uncover her.

Good Luck!