The crackle sound in your knees when bending or walking up the stairs could be due to air popping because of the synovial fluid that is found between the joints. It could also be crepitus which is a symptom of cartilage wear in the joint space. You can visit your doctor for more details.
Because thats what happens to alot of people. To remedy, go to your local herbal medicine shop and ask the knowledgable attendant what minerals or herbs to take for lubricating ones joints and sockets. Goodluck.