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Strange popping in left shoulder when doing swimmers?

For some reason, whenever I do swimmers or even back bows, my left shoulder has a strange popping noise/feel. I know Kelli says, "It's ok if you feel popping, as long as there's no pain" but it's honestly just annoying, and it's been happening for a while and I don't want to hurt anything. Does anyone know why this could be happening? Thanks!