I just took my cat to the vet for this today - very disturbing. The crunchy or crackly feeling (and sound) is likely caused by air trapped under the skin (sub-dermal emphysema) which itself can be caused by a couple of different things. In our case, my cat had a deep puncture wound at the base of his tail from a fight, which allowed air to move under the skin a few inches away from the wound, which is where I felt the 'crunchiness'. The vet had been very concerned by the symptom, because it can also be caused by damage to the trachea or a collapsed lung, either of which would allow air to move from the lungs to the inside of the body, and possibly under the skin.
If you feel this while petting your cat (or dog, or family member or whoever), get them medical help immediately. Good luck!
If you feel this while petting your cat (or dog, or family member or whoever), get them medical help immediately. Good luck!