I use to get this and it freaked me out the first few times...
its normal, although nobody could tell me exactly what it was...
a few suggestions that i was given were
joints clicking
bubs actualy kicking something in you that makes the sound
ligaments moving
as they are moving the sac moves too and if it folds over no fluid is in there, then with another movement it quickly fills again...
these are all pretty random explainations, but rest assured its normal (or it was in me )