What is the natural sources of noise?

All sources of noise are natural. Noise is otherwise defined as soundwaves, which vibrate through matter until they reach your eardrums and the energy used to cause the vibrations is translated into noise by your brain -the pitch is determined by the frequency of the vibrations-. Anyway, it does not matter how the soundwaves are created, it is all natural. You cannot create synthetic noises as far as the physics are concerned.