What can cause your feet and ankles to constantly pop?

2 options. Some cases are due to the prolonged build up of certain gases that are able to escape the joint once a certain pressure is reached and movement is occurring. Arthritis will cause similar symptoms with a more frequent occurrence, happening repeatedly after seconds rather than every 20-30 minutes.
Tissue pliability. Popping of ankles and feet, if not painful in and of itself does not indicate anything. However it may represent increased range of motion of the joint's in question. Strengthening the muscles around the ankle would decrease this popping, by limiting the active range to a functional one.