I'm assuming you had imaging to diagnose the sacral stress reaction?
Did an ortho diagnose you? If so have you are are you currently seeing either a PT or Chiro?
My next question is, how do you know that the snap is an "adductor snap"? If you are hearing an audible snap/pop from the area of your pubic bone, that it's the symphysis itself that is making the noise.
Let me ask you this, can you feel anything directly afterward? Is there a light stinging or slight burning feeling right in the center? It should not be drastic at all, but if you go to adduct again right after the snapping noise it won't feel comfortable.
My guess is that your adductors are affected because if your Ilium is not properly aligned (usually resulting from very small degrees of shifting at the pubic symphisis and/or SI joint) then it can dramatically change areas like your groin. For instance, lets say you shifted anteriorly on your affected side (your Ilium tipped forward past the healthy alignment of 10 degrees). This is going to create a kinetic chain of events including shortening of the adductors, and inhibit recruitment of your glute max and hamstring.
The adductors is one of the first things you'll notice because the groin is extremely sensitive, especially when put on stretch. I'm going to guess you have tried stretching your groin? Did it respond positively and relax or did it just fight the stretch and remain somewhat contracted? It doesn't have to be in a spasm, it's just a general sense of discomfort and tightness that you can't shake.
What I would do is see someone to evaluate your pelvic alignment. You obviously had trauma to the sacrum. There is a huge chance that you have developed a misalignment issue (it may only be a few mm's which is very common). You can try to check for yourself by noting your spinous processes on the Iliums. They should be lined up nicely from side to side. The biggest landmarks are the ASIS, PSIS, and then also the Iliac Crest...depending on the degree of rotation it can be hard to tell or extremely obvious and you can see it by simply lifting up your shirt.