it's like a tiny pop noise right?  when i rotate my legs in a certain position i can make it click every time.  it'll also make the sound if i flex the toe up while squeezing/flex the whole leg, or when i simple squat down ATG while touching both knees together.

one massage therapist explained that it is air trapped in the tendon sometimes and the sound is just the air getting squeezed out and it's nothing to worry about.  assuming we're talking about the same noise.