When the leg is stretched and curled, it may be related to the following reasons: 1. There may be lesions of the muscles and tendons outside the knee joint. For example, most often there is a spasm of the iliotibial bundle under the lateral thigh, often due to contracture of the tendons on the outside of the knee joint as the patient sits or stands for long periods of time. When the patient straightens the knee joint, the tendon slides over the bony prominence and then produces a strange sound. 2. There may be an injury inside the knee joint. For example, the cartilage, meniscus, and ligaments inside the knee joint may be damaged for various reasons, resulting in a knee noise when the lower extremity is straightened. 3. There may be inflammation of the joint inside the knee joint. For example, there may be osteoarthritis, rheumatoid arthritis, gouty arthritis, etc. of the knee joint. This type of joint inflammation can lead to a series of internal damage to the knee joint, which in turn leads to knee ringing when the patient moves the lower limb. In addition, the above-mentioned lesions of the hip and ankle joints may also cause the ringing condition.4. It may also be caused by the free body in the joint.