I have been having the same sensation but I was told that it was the baby's ligaments and joints popping, although I am not sure if this is exactly what you are describing.
I also feel the popping, I asked my doctor and she said it was normal baby is moving down toward your cervix and also feel like bubbles passing down.
It feels like I'm putting fitting something togethee. It's quiet a weird feeling to describe. My cervix felt bruised before this happened and someone said its him moving his head down. Have mw again on Wednesday unless I have given birth by then so we shall see. Just want him here now.