Originally Posted by Trouble

It's air trapped within the cartilege in your rib cage.

Its a harmless condition. Some also hear the same kind of crackling in shoulder and knee joints a well.

Originally Posted by DLight03

I realize this is an old thread, but I figured it would be an opportune place to ask a related question:

What if this sound is coming from over your heart rather than the lower middle of your chest? Is this no different or something I should be concerned about? Not much of a difference in distance, but I figure it's important to ask.

Also, the sound I am referring to can be best described as velcro being undone, which I think is more accurate than crackling