When I was diagnosed with tendinitis in my shoulder, I heard lots of popping and grinding in my shoulder when I was doing physical therapy, but you could not really hear it unless it was real quiet in the room. PT guy said it was scar tissue and would go away as stuff healed. So far so good. When I was diagnosed with arthritis in my retinacular band, my knees would pop and crack so loud people across the room would cringe. This fits the description of the stuff in Rusty Guy's last link. I would feel pain before and during the pop, but great relief after wards. The surgery I had was called a lateral release. I had lived with the problem for many years, escalating medication until I finally got a cortisone shot. The one day of relief I got from that convinced me how bad knee had become. Surgery fixed the problem. Your mileage may vary.