According to a Chicago Tribune article by the Mayo Clinic, the crunching sound from a knee is called crepitus. Crepitus is caused by the rubbing of cartilage on the surface of the joint or soft tissue around the knee. If there are no other symptoms, then no specific treatment is required.
If pain accompanies the crunching sounds, its source is usually scar tissue, a meniscus tear or a tendon moving over a bony prominence located within the knee joint. Crepitus usually occurs when squatting, using stairs or getting out of a chair.
While crepitus is not a concern, cartilage can become rough and wear down, leading to arthritis. Strengthening the quadriceps muscle by walking, biking and swimming helps to reduce the risk of additional knee problems. A doctor or physical therapist can suggest additional exercises with or without weights that can be beneficial. Left untreated, knee conditions can lead to pain, joint damage and disability, according to the Chicago Tribune.
According to the Mayo Clinic, symptoms that can accompany knee pain include swelling, stiffness, redness, weakness, popping or crunching noises and an inability to straighten the knee fully. A patient should seek medical attention if the knee is unstable, cannot bear weight or has a visible deformity.