The cause of a ringing sound during knee flexion and extension may vary by age. If the knee joint is flexed and extended in a young person with a significant rattling sound, it is most likely due to a meniscus injury. If there is a combination of these phenomena, there is a high probability of meniscal injury, and the diagnosis can be confirmed by MRI. In the case of middle-aged and elderly people, a ringing sound when flexing or extending the knee joint is usually due to degenerative osteoarthritis of the knee joint or a free body of the knee joint. For less severe degenerative osteoarthritis of the knee, oral glucosamine can be given; for severe osteoarthritis of the knee or free body, arthroscopic cleaning can be performed; for particularly severe osteoarthritis of the knee, joint replacement surgery should be considered.