Chattering is a constant, low-pitched sound, like the noise small animals make. The chattering of the squirrels in your yard might wake you up before your alarm goes off. Be quiet, rodents!
The chattering of kids in the cafeteria may become louder and louder through the lunch period, while the chattering of chipmunks on your favorite hiking trail is barely audible. Sounds that machines make are sometimes described as chattering too, like the chattering of your brother's old typewriter as he works on his never-ending novel. When someone wants to insult academics and educated people, they might refer to them as "the chattering classes."