MCL Pain
In addition to fairly severe pain and swelling, a torn medial collateral ligament (MCL) [3] in your knee may also produce an audible popping sound. Whether you strain it or really tear it will affect how painful it is. Afterwards, you’ll probably experience some stiffness and edema, and your knee may become immobile.
Cartilage injury
Your knee may pop as it goes back and forth if a portion of cartilage has become injured.