What has happened immediatly prior to the outbursts , is there just you in the room with her, it sounds as if something happened before the screaming, do you think she is terrified or does she seem as if its a habit at hearing the noise she can make ?
Perhaps she is having a tummy ache? I would mention these episodes to her doctor. Kids of that age can definately throw a tantrum that includes screaming but they usually look mad and thrash around a bit if it is a tantrum. Good luck!!
She sounds like she is having hallucinations. Have you spoken with her doctor yet about it? They may be able to help you with her. Or at least give you some insight as to what to do with her.