The tongue of a bell — the part inside that swings to make the bell ring — is called the clapper. The Liberty Bell cracked the first time the clapper struck the sides of the bell. The bell was recast twice, but eventually the clapper cracked that bell as well.
The noun clapper is based on clap, from the Old English word clappan, which means to clap, throb, or beat. The word is supposed to be imitative of the sound the thing makes. When the clapper strikes the inside of a bell, it makes the bell ring. If you need the bell to be silent, you have to muffle the clapper to keep it from striking the inside of the bell and making a sound.