This Does Not Seem To Be A Serious Issue
The crack may be comming from the C-3 area of the neck (spinal column). This is the area where cervical nerves (C3, C4, and C5) roots innervate the diaphragm and other respiratory muscles, are located.
The crack may just be the as a result of synovial fluid, which is used in every joint as a lubercant, to make movements smooth. ple, when people pop their knuckles, it is usually caused by the loint sliding back in place, with little or no s. fluid.
This coupled with respiration (inhale) could allow for the muscles and ligaments connecting the diaphram and the neck (C3) joints to shift or pull. The effect would be the sound of a knuckle cracking.
Again, it does not appear to be anything life threatening. But if, however, you have pain in the future i.e. arthritis in the neck, this is probably the area where it will originate.
Hope this answers your question!
1. Transverse Myelitis Association, Map of Spinal Column Online: TMA, 2005) http://www.makoa.org/scimap.htm
2. John H. Bland, Clinical Approach to Neck Pain:
Diagnosis and Management (Online: American College of Rheumatology, 2005)