The sound you hear when macho people crack their knuckles is actually the sound of ------- gas bubbles bursting.

The cracking sound is produced by nitrogen gas bubbles bursting in the synovial fluid between joints. When you crack your knuckles, you're rapidly creating negative pressure that causes dissolved nitrogen to form bubbles and collapse. Oxygen and carbon dioxide are also dissolved in the fluid, but nitrogen makes up most of the gas and is primarily responsible for the sound.