Popping sound in achilles/top of heel

Hi all,

So I'm doing eccentric exercises as part of achilles tendinitis rehab as prescribed by my PT. On the lowering portion of the exercise (on one leg), there is a somewhat small/quiet popping noise right at the back/top of my heel. It happens on every rep. Any thoughts as to what this might be?

it might be one of the few tendons that pass around under your ankle bones just popping out a little and back in. happens quite often. i wouldnt worry too much at all if its not painful.