The rattling sound in the knee joint is commonly caused by the following reasons: a. Osteoarthritis of the knee joint, due to degeneration and hyperplasia of the joint, which causes friction and impact between the hyperplastic bone or deformed bone, or friction and impact with the surrounding soft tissues when moving. Synovitis of the knee joint, due to the hyperplasia and hypertrophy of the synovial membrane, there will also be friction and impact when moving, resulting in a rattling sound. Third, meniscal disease, including meniscal injury and meniscal degeneration, may cause a sound due to impact and friction when moving. In this case, you must go to the hospital for a detailed examination. If left untreated and untreated, it will lead to chronic strain and injury of the knee joint and wear and tear of the knee cartilage, resulting in serious deformation and dysfunction of the knee joint, so it should be diagnosed and treated early.