Please get this checked i dont want to worrie you it coukd be many things but my friend had the same sort of thing it turned out the baby was caught up in cord making it pull everytime baby moved or she led on her side she had to have a c section baby was fine. X
I'm 39 weeks and I've never hears this with my baby BUT I've read it happens to some woman and their pregnancy.
It Could be your babys hiccups.
I've had something similar to this, it sounds like kind of a popping noise like cracking knuckles. I have no idea what it is either.