What my joints do if I haven't stretched in a while. Or just if they feel like it. But also what bubble-wrap does when put to its proper use, which to be therapeutically popped (I've no idea what shippers and packers are thinking). The word has also been used to describe the sound of a rattlesnake, though I think of that as rattling too rapidly to call it a crackle. This cellophane-crinkle of a word was adopted around 1620 from Latin crepitātus, the past participle crepitāre, to rattle/rustle/clatter/chatter, frequentative of crepāre, to creak/crash/break with a noise. For a usage example, I give a review by Gene Weingarten:
"John Grisham’s sentences thud and crepitate all over the page."