The common causes of bent knee ringing are physiological popping, meniscus injury, free body and chondromalacia patella and other diseases. 1. Physiological popping: because the knee joint is large and in a relatively negative pressure state, so when the knee joint activities will make its pressure changes, can cause pressure release burst sound, that is, produce physiological popping. 2. Meniscus disease: meniscus has the role of stabilizing the knee joint and increasing the contact area of the joint. If the meniscus is damaged or there are other meniscus diseases, knee activities may bring about a rattle due to meniscus pulling. 3. Knee joint free body: Meniscus injury, bone injury and cartilage injury can cause small pieces of bone or meniscus detachment at the injury site, which will form a free body in the joint. When the joint is flexed, the impact of the free body will cause pain and popping sound. 4. Chondromalacia patella: Chondromalacia patella can lead to uneven patellar joint surface, when bending the knee joint, its joint surface and femoral joint or the surrounding soft tissues will produce friction, and there may be ringing sound. Therefore, when bending the knee ringing, it is recommended to go to the hospital for examination to exclude pathological causes.