Gorillas make quite a lot of noises (around 20 I once read), but most of them are so soft you won't hear them in zoos. The only exception is screaming. That happens when females have an argument about food or you hear it when a youngster is handled to rough by an older sibling. Because of the sound of that scream, you can hear it through windows (the screams of adults better than those of a youngster of course).
Another sound they make now and then is coughing. Most of the time a gorilla comes too close to another one and with this cough the gorilla warns the one approaching to stay away. Again: in most zoos you don't hear it. Only when there are no visitors in Artis I can hear it there (because the windows are not against the ceiling). There, in Artis, you can also hear some other sounds. The most special one is the 'singing' when the gorillas are in their night cages and the keepers put food down in their enclosure. I really love that sound and treasure rainy week days when it's not crowded in Artis. Chest beats also can be heard in Artis now and then and in other zoos when they're outside. The singing and other sounds also can be heard in Apenheul when the gorillas are inside. They have speakers in the visitors part so you can hear the gorillas (sometimes the volume is too loud unfortunately). When I'd anything to say about it, those speakers would be put in more gorilla buildings (but than on a normal volume).
The most special sound of a gorilla (at least, that's my opinion) can't be heard behind any window. It's a sort of soft, short 'growl'. That's a sound they make when they're satisfied. I twice had the change to hear that from a gorilla on a few centimetres distance (I had the opportunity to go backstage in two zoos) and I was really overwhelmed by it. To hear that gorilla greeting me in that way … I still get goose flesh when I think of it, even now. It was the best thing ever. People who visited gorillas in the wild, know that sound too.
Many people say 'ooh ooh ah ah' when they look at the gorillas but that's a sound I never heard coming out of the gorillas (or any other ape/monkey).