To rattle is to make a very rapid, short series of knocking or tapping sounds. You might impatiently rattle the quarters in your pocket as you await your turn at the pinball machine.
Ice cubes rattle in your glass of lemonade, and beads rattle in a jar. Something else that rattles is a baby's rattle, a noisy toy that's filled with small balls or pellets. Figuratively, to rattle someone is to upset or irritate them, like the way a sudden thunderstorm might rattle your timid poodle. Experts trace this word back to the Middle Dutch ratelen, which they believe to be imitative since it sounds a bit like a rattle.