Occasionally hearing pops, snaps, and crackles when you bend your knees is known as crepitus. Over time, gas can build up in the areas surrounding the joint, forming tiny bubbles in the synovial fluid. When you bend your knee, some of the bubbles burst, and ligaments may snap or pop. This is normal, and happens to everyone from time to time. Knee crepitus can happen at any age, but it is more common as people get older. It can affect one or both knees.

Arthritis is very common but is not well understood. Actually, “arthritis” is not a single disease; it is an informal way of referring to joint pain or joint disease.

Cavitation is the formation of a cavity, in the lung as in tuberculosis or the development of a bacterial abscess.

Tendonitis is an inflammation of the tendon, a flexible band of tissue that connects the muscles to the bones. The pull of the muscles is transmitted to the bone by the tendons, which allows movement. When the tendons are inflamed, the pulling action of the muscle is impaired and movement becomes painful.

More Info: en.wikipedia.org