This is probably her character and how she is, you will probably never change her until she meets another rat that is a bigger bully than her. But... if there are no cuts or bites or blood, I don't think I would worry about it too much.
A couple of years ago, I adopted two brothers, they were already 12 months old. Vincent was always screaming and Pascal was always on top of him. I thought I would have to separate the two of them. I wasn't happy. But one night, I decided to hide behind the couch and watch to see what was going on and why Vincent was screaming like a banshee. This is what I saw: Pascal was sitting minding his own business looking out the cage near the door. Vincent would climb the ladder to the shelf just on top of Pascal. He then would jump on Pascal, Pascal would quickly wrestle him and put him on his back all the while screaming bloody murder. Then he would stop... climb up the ladder and jump on Pascal again. That's when I realized it was just playing.
So sometimes those screams are actually squeals of joy.
