Cockatiels can be quite loud. Mine always seem to crank up the volume when I'm on an important phone call Wish I had some answers, but it sounds like he's asking you for more attention, which may or may not be warranted. I do think ours settled down some once we got him a partner. Of course, he also stopped learning anything new because we weren't as important to him. I do find that ours settle down in the early evening and sleep for at least 12 hours.

As I'm typing, mine is talking his head off! LOL!