What causes lowback tailbone pain with a cracking joint sound?

Low back pain. Low back/tailbone pain can be caused by numerous conditions including disc degeneration, herniated discs, facet joint degeneration, si joint pathology, etc. The cracking sounds are generally benign, usually caused by the spinal joints themselves (like cracking your knuckles) or by inflammation around the spine. If your pain is severe or has been longstanding then you should see a physician.
Tailbone pain. Consider coccydynia, s-i dysfunction, and referred ls pain.