They do it often, not just on the presence of Ms. Goodall and her monkey talking. The first time we heard it was in December when DH and I were in the park at rope drop to ride EE a few times. We headed over that way to see what all the noise was about and a CM told us they did the every morning. There are two different types of monkeys and they scream like that to establish territory (although they are separated already) It sounds crazy and freaks my 2 year old out! We heard it again in early Feb and in March when we took the kids to Tuskar house for breakfast, both times we were there for rope drop. From what I understand it only happens in the morning after they bring them out. My son loves it an now makes "territory calls" when his sisters go in his room.... Just thought I would shed some light on the mysterious monkey screaming!