The sound you hear when cracking a knuckle is caused by “popping” bubbles in the synovial fluid, the fluid that lubricates your joints. The sound you hear is likely gas bubbles in the joint bursting.
Repeated joint popping can also be caused by conditions that represent more serious joint status, such as a cartilage tear from a torn meniscus or degenerated cartilage from arthritis (osteoarthritis). You should consult with an orthopedic doctor and get it treated