It occurred to me today, while I was thinking about alliteration, that the phrase "cracking knuckles" is wonderfully onomatapoeic. All those wonderful K sounds mimic the actual sounds of the cracking knuckles themselves.
Of course, crack (the sound, not the splittage or the illicit drug) is already a bonafide onomatapoeia, so "cracking knuckles" is, I guess, just an extension of that characteristic. Saying it in a steady, accented rhythm really brings out the sounds, too: KRAK-KING-NUK-KULLS
This is all just simply observation. Language is fun.