The Fox sisters. These young women were the founders of the spiritualist movement, which was very popular in the mid and late 19th century. In 1848, they (at the time around their early teens or slightly younger) began claiming that they could communicate with the dead. Loud knocks were the sign. Soon, mediums popped up around the country. This proved particularly popular in the years after the Civil War, with so many grieving families desperate to communicate with their dead.
Later in life, 2 of the Fox sisters admitted that they produced those loud knocks by cracking their toe joints, which I guess they could very well.