(Renato Augusto Martins/Wikimedia Commons (CC BY-SA 4.0))
Anything. The noise is generated by the beating and friction between the very parts that make up the end of the rattlesnake’s tail, called rattles. There are no stones or balls that stir inside them. – like that rattle you make in preschool out of sand and yogurt pots.
The rattles are hollow and made of keratin, the same rigid protein that forms our fingernails, horse hooves and deer antlers (hence the hardness being enough to use them as a percussion instrument).
The rattlesnake is born without any rattles. They form during shedding, which occurs three or four times a year in the wild (and sometimes more in captivity).
The old fur sheds from head to tail, and turns inside out as it comes off – just like when you pull off a sock from the ankle. Upon reaching the end, a section of skin is trapped in the first rattle, forming a new rattle below it.
This makes it possible to estimate the snake’s age from the number of bells, although not with absolute accuracy. A snake with nine rings on its rattle is probably somewhere between two and three years old. The rhythm of skin changes is influenced by issues such as environment, health, age, climate and food.
The rattle tip is smaller because the oldest rings were created when the snake was young and small – and the animal can lose part of the rings throughout its life due to predator attacks.
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Sources: Giuseppe Puorto, biologist and director of the Biological Museum of the Butantan Institute, Luiz Antônio B. de Mello Lula from the São Paulo Zoological Park Foundation.