Many people’s knees squeak, crack, or crackle, a phenomenon known as crepitus. Most of the time, it’s no big deal at all. But a loud popping sound accompanied by pain and swelling indicates an injury.

The sternum, or breastbone, is the flat bone where the ribs meet. Sometimes, the joints can pop or crack as a person stretches or moves. A muscle spasm, an injury, costochondritis, and surgery are ...