I've heard these noises when my baby kicks as well, like the sound of joints popping, like when you crack your knuckles. The only thing I can figure is that they're bubbles in the amniotic fluid popping.
It's not for certain, but my midwife said that as best anyone knows, it's the two layers of the amniotic sac building up tension and then snapping past each other, if that makes sense. It's normal though.
Re: Baby Kicks that make Popping sounds