I get this sometimes & I assume it's anxiety related. Also, the more attention I give it the worse it seems. It happens sometimes when I'm trying to fall asleep & freaks me out, so I change my position & try to ignore it. Sometimes I repeat something in my head (for whatever reason the "Our Father" prayer pops into my head a lot) over & over to try to get my mind off of this & fall asleep. (Kind of like counting sheep, I guess!) It always works!
Also, I've had my heart & lungs checked fairly recently (x-ray, EKG, bloodwork, etc.), so I don't feel like it's anything to worry about. It is freaky, though!!!