What does Scout reveal that she heard as she was rolling into the Radleys' front yard in To Kill a Mockingbird?

When Jem, Scout and Dill are playing their game about the
Radleys, Scout is not really into it. She tells Jem that Atticus doesn't want them to
play the game anymore, but Jem and Dill still do so. Scout is reminded of what she heard
when she rode the tire into the Radley front yard. She is still unnerved by all of this
and just wants to stop playing, so when Atticus comes home she is glad that they have to
stop so they won't be caught.

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Atticus's arrival was the second reason I wanted
to quit the game. The first reason happened the day I rolled into the Radley front yard.
Through all the head-shaking, quelling of nausea and Jem-yelling, I had heard another
sound, so low I could not have heard it from the sidewalk. Someone inside the house was
laughing.

What Scout doesn't
realize is that Boo was watching the children play. Scout is certain of what she heard,
but she is still confused and rattled by what she had heard and what it means. Little
does Scout realize, she will be ever so grateful that Boo has been watching them, and
she will have him to thank for their lives.