Hello PhysioRoz,
I agree with Yarok that the most likely reason for the pop will be a stressed meniscus. Sometimes a patello-femoral malalignment can create a click upon release
when the quads are in a tightened or protective contaction position. The only other pops can be synovial fluid vacuum pop or a bakers cyst - unlikely.
The situation should be self resolving unless a meniscal tear has occurred and given resolution of the situation is also unlikely.
Advice: avoid extreme flexion rotation of the knee during this exercise in future.
Cheers,
Mark Quittner MrPhysio+