In the end of chapter 4, the kids are playing the "Boo Radley" game. They seem to get in trouble with Atticus because he reprimands them. Scout experiences a sense of relief and then reveals to the audience something she had experienced earlier when she had been spit out of that tire in the Radley's yard. This is very important because it reveals in only one sentence something that she thought occured and was pretty sure about: Scout heard a noise in the Radley's house. This was the noise of someone laughing. This laugh she heard was a really low and difficult to hear noise and she feared telling the other kids about it, but she has confidence enough in readers to reveal to us her secret.
This leads the reader to believe that maybe it was Boo in the house laughing, but she does not in any way confirm identify for the laugh.