http://www.minti.com/parenting-advice/231/Screaming-How-to-make-it-stop/

This is a great article that covers screaming let me know if it helps

I had that times with my son when he was around 2-3 years old. I found it sort of like frustation or something. So, when it happened I usually, first calm my self, (take her to bedroom if you could) I cuddled him tightly without hurting him and tried calm him down. Spoke to him calmly, and asked him to take a deep breath to calm and speak quietly cause I said that I wasn't deaf and would be better if he could speak in normal voice. And I'd asked what did he want or what happened. It might take time or probably other people do other way. I just thought if they scream and we scream too, it'd just ended everybody scream. So, I start with myself showing to speak quietly. Good luck!

For screaming I touch my daughters lips with one finger and say "You must stop screaming"  if that doesn't work immediatley then I repeat and place her in time-out.  Do that every time and eventually your child will get it.

Has it been a habit for a while (2 or more months?) or just something that may pass with time?

Hi, I wondered what you were doing to discipline you kids and what age they are?