Ghost at Dawn's House, The (Babysitter's Club #9) by Ann M. Martin

Dawn has always thought there was a secret passage hidden in her house. But she never thought there was a ghost... until now. All kinds of creepy things go on whenever Dawn's at home. There are even spooky noises behind her bedroom wall!

Dawn is sure there's a ghost in her house. And so are the other Baby-sitters. But they're so busy with their baby-sitting jobs that they hardly have time for a ghost hunt. Will Dawn and her friends ever solve the mystery, or will Dawn have to share her house... with a ghost?