My kneecap makes some kind of noise when walking. what could it be?

Degeneration: One of the most common causes of grinding/noise in the knee is chondromalacia, which basically refers to degeneration (wear and tear/aging) of the joint surface of the knee cap. Degeneration in other parts of the knee can also cause similar symptoms. At your age, physical therapy to optimize joint function and strength may be adequate treatment.