There are many reasons why the knee joint is rattling, such as osteoarthritis, synovitis and meniscus injury. 1. Osteoarthritis: due to a variety of reasons leading to the proliferation and degeneration of the knee joint, friction and impact between the proliferation or deformation of the bone during the activity, thus appearing knee joints rattling performance. 2. Synovitis: due to injury or other reasons, the synovial membrane of the knee joint appears to be hyperplasia, hypertrophy, congestion, etc., which will cause the knee joints to rattle due to friction and extrusion during activities. 3. Meniscus injury: when the meniscus is damaged due to external impacts, the knee joints may be rattled due to the abnormal extrusion and friction between the femoral condyles and the torn meniscus during the activities. There are many other reasons that can lead to the phenomenon of knee joints rattling, it is recommended to go to the regular hospital, consult a professional doctor, complete the relevant examination, to clarify the cause, to avoid blind treatment.