Sounds alarming I know. But if I lie on my back and twist my legs over to the side, then lift my knee to my chest. There is a crunching sound and sensation in my lower back. I've recently started Pilates for instability and bulging discs in my back and this "crunching" sensation is quite new. There's not much pain involved when it does this. But, as you can imagine, it's really unnerving lol. If anyone knows anything I'd appreciate any information you may have.