I always thought it was to do with the clappers of a bell but found these refs in Chambers...

"that which claps, as the tongue of a bell: a contrivance for shaking a mill hopper (I like this better than the bell thing): an instrument for making a noise, as a rattle or (plural) bones for keeping musical time: the tongue (slang)." (Now that's given us a fair old crop to vote upon!!)

stales