The possible causes of knee rattling 40 days after hollow nailing of femoral neck fracture include prolonged inactivity, tendon popping, and chronic inflammation of the joint. 1. Prolonged inactivity: after femoral neck fracture surgery, the limb is inactive for a long time, which leads to the reduction of lubricating fluid produced between joints, increasing the friction between joints, and a ringing sound may be heard in the knee. 2. Tendon popping: it may be localized tendon hyperplasia, which causes tendon popping caused by crossing the bone when moving. 3. Chronic inflammation of joints: It may be due to chronic inflammation of joints caused by chronic strain injury, which causes adhesion of muscle tissues, so when the joints are moving, there is a ringing sound. There may be other reasons for the ringing in the knee 40 days after the fracture of the neck of the femur with a hollow nail, it is recommended to go to the hospital in time to improve the examination, to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.