As we age, our cartilage decreases. This opens a space in the joint, which can fill with air, which makes the "pop" sound with movement. Also the tendons can then become strained with any movement that is not in line with the bones.

Also the joint may become unstable, as it loosens. A knee brace may be a good idea if you feel your knee is unstable. Don't get a real tight one... too tight and it compresses the nerves behind the knee...the peroneal which is sensitive and leads to foot drop if damaged. There are good braces that adjust with velcro straps now.

you may have to have an orthopedic doctor evaluate your knee before
it slips and you get a dislocation, which is very painful and hard to heal up.