"When an ankle is sprained, a peroneal ligament can be torn. Over time it can flop out of position, causing a clicking sound, often when the ankle is flexed, as when going up or down stairs.
"Usually patients can reproduce the sound on request, and sometimes you can see the tendon pop around the ankle joint. If it isn't painful and doesn't bother you, you don't need to do anything about it, but if it becomes painful you should look into it before the tendon becomes irritated or even degenerates. Surgery can put it back in its place.