In Lee's To Kill a Mockingbird, what might be the cause of the laughter from inside the house?
As the previous answer states, the laughter from inside the house is probably Boo Radley having a good chuckle as he watches the antics of the children outside his door. Scout never lets on to Jem and Dill that she heard it at all; she keeps it to herself. This is probably because she is too afraid to even mention it. At this early stage in the novel, anything to do with the Radley house scares the children, certainly Scout, who is the youngest. The laughter seems scary to her.
It is not until the end of the novel, when Boo Radley is finally revealed for the eminently decent person that he is in saving the children from the depraved Bob Ewell, that Scout's perspective about him undergoes a sea-change. We get hints before this that the older and wiser Jem is already changing his ideas about Boo: for instance when he remarks thoughtfully that Boo's seclusion inside the house, hitherto such an intriguing mystery to the children, might be simply 'because he wants to stay inside'.
But for Scout, the revelation about Boo Radley is withheld until the end of the story. Then, it happens in a flash, and she comes to understand him and to literally see things from his perspective as she stands on the Radley porch looking down the street. She realizes now that Boo, or to give him his proper name, Arthur, has always been watching them and looking out for them, and that they also unwittingly helped him by affording him innocent amusement as he watched all their games. The laughter that Scout hears from the house earlier is an indication of the happiness that he derives from the innocent children playing in his neighbourhood. Although she doesn't know it then, it is a positive sign.
Boo Radley spends every day in the house; as a result, he is probably often bored. When a tire comes rolling uncontrollably towards his house in Chapter 4, it catches Boo's attention from behind the dark windows and curtains. This event causes him to laugh from inside the house and can best be expressed in the following passage:
"The tire bumped on gravel, skeetered across the road, crashed into a barrier and popped me like a cork onto pavement. Dizzy and nauseated, I lay on the cement and shook my head still, pounded my ears to silence, and heard Jem's voice: 'Scout, get away from there, come on!'" (37).
This must have been quite a comical stunt to watch because Scout says she popped out of the tire "like a cork." Boo Radley must have found Scout's experience with the tire funny and therefore chuckled because of it. Scout doesn't admit to hearing the laughter from inside the house to Jem or Dill, though. Hearing the laughter causes Scout not to have the desire to play Jem's reenactment game of the Radley family saga in their front yard. In fact, hearing someone laugh from inside one of the spookiest houses in Maycomb makes Scout more afraid of the legend of Boo Radley. It means that she has verification that Boo is still alive and not dead and stuffed up the chimney like Jem thinks. Based on the rumors that circulate about Boo Radley, Scout also thinks that he will come to her house during the night to do something bad to her if she and the boys keep playing the game.
At the end of chapter 4, Scout admits in her narration both of the reasons why she wanted to quit playing the Radley game. The first was because Atticus showed up and they didn't want to get into trouble. Her second reason refers back to the day when Jem pushed her in the tire. He was angry with her for contradicting him on "hot steams." He pushed her a little too hard and she ended up on Radley's front porch.
"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."
Although Scout may think of the laughter as spooky, Arthur was most likely inside watching their every move. How entertaining it must have been to watch Jem push Scout (a little too hard) down the road and watch her pop out of that tire all confused and disoriented. Most likely his laughter was pure amusement in a normally boring town.