Sometimes I sound like a bowl of Rice Krispies with my "snap crackle pop!"
Technically, it is the peroneal tendons slipping in and out of the groove behind the bone sticking out on the outside of the ankle. Medically, it’s known as peroneal subluxation. It is caused by rolling your ankle when you run or from other sports injuries, sprains, etc.
Mine do it from old sports injuries incurred during football, basketball, track and baseball.
Doing Mark's mobility exercises for the ankles really cuts down on the clicking noise. Also, walking barefoot or in VFF's as much as possible has significantly reduced the occurrence. Upping my fat has probably helped as well. The main thing I do is the ankle mobility exercises and riding bike also seems to help get things moving/flowing.