The knee may ring when it moves because of the following reasons: 1. injury to the meniscus, after the meniscus is injured, there will be localized meniscal stumbling, which will occur when the knee joint moves, accompanied by pain. 2. it may be a free body in the knee joint, when the free body is stuck in the knee joint cavity at a certain time, the joint cannot move, and it needs to move slowly to start normal activities, which will also occur. 3. The knee joint may have a popping sound due to a bad patellar track, patellofemoral arthritis, and popping sound when the patella is moving. 4. The ligaments around the knee joint or the synovial folds inside the knee joint may also cause popping sound. In addition, there may also be some unexplained popping, the details of which can only be further clarified after a hospital visit.