There are several common reasons for the popping sound when the knee joint is bent: 1. Physiological popping. The knee is a relatively large joint and is relatively negative pressure state. When the knee joint is active, there will be a change in pressure, resulting in a pressure-release bursting sound that produces a physiological popping sound. 2. Meniscal disease of the knee joint. The meniscus has the function of increasing the contact area of the joint and stabilizing the knee joint. If the meniscus is damaged or diseased, when the knee joint moves, a ringing sound occurs due to the vibration of the meniscus. 3. Free bodies in the knee joint. Whether it is meniscal damage, bone damage or cartilage damage, resulting in the loss of small pieces of bone or tissue at the injury site, forming free bodies in the joint, when the joint is bent, the collision of free bodies will produce pain or popping. 4, patellar chondromalacia, patellar chondromalacia leads to uneven patellar joint surface, when bending the knee joint, the joint surface rubs against the femoral joint or the surrounding soft tissue, resulting in a rattling sound.