You probably have osteoarthritis of the hip which is more common with this joint than rheumatoid arthritis. In osteoarthritis the cartilage that lines the end of the bones degenerate. It creates a rough surface which then rubs against each other with movement. Eventually the cartilage wears out entirely and the bony ends rub against each other. Cracking or grating of the joint is therefore a common feature of osteoarthritis but more so in the advanced stages of osteoarthritis. Cracking of a joint may also be due to gas bubbles popping in the joint which is known as cavitations.