Pain builds up, muscles popping when stretched, pain gone

Over the last few months I'm experiencing the following: the muscles in my inner upper thigh/groin and upper hamstrings seem to randomly build up some kind of nagging pain, this pain is not debilitating, just a bit nagging.

When I stretch them out, especially those near the groin/inner thigh, I hear a 'pop' sound which doesn't hurt after which the nagging pain is gone. I can't recreate the popping then till a few hours later, and not always... it can be hours, it can be a day before I can do it again.

Any idea what this is? It's definitely not the hip joint itself, it really feels like it's tendons or muscles becoming extremely tight, or something builds up, which then 'pop' to length somehow... not sure this makes sense.

Any scientific explanation for this?

Sounds like a popping from the public symphysis area where the adductors etc attach aroudn the groin. Squeezing a cushion between the inner thighs can often get this pop and release. It is probably a generalised imbalance in your musculature and some might say some instability in your pelvis. Sometimes also it can be a boney instability in the very low back which can also feel that the pop is coming from the gropin. It's very likely nothing to worry about. Go see someone for an assessment and program of stretching and strengthening. On one off assessment and maybe a followup after 6 weeks of doing the program should suffice.