What is the rattle when the left knee moves?

This bounce may be caused by the vibration of gas in the joint cavity.

The gap in the joint cavity is filled with liquid, so that the joint can work smoothly. When the joint moves, there is an obvious cavity in the middle of the joint cavity. The surrounding gas will diffuse into the cavity and make a crisp sound with the liquid.

If the doctor can confirm that there is no joint disease, then it is simply breaking the knee. No pain, little impact on the human body, don't worry.

But if the knee joint feels obvious pain when doing squats and other actions, it may be a pathological snapping sound.

Pathological snapping is generally caused by facet dislocation, local ligament calcification and articular cartilage injury. That is what we usually say, osteoarthritis.