Noise is generated within ckt elements such as resistors etc. The one generated in resistors is also known as shot noise. Now suppose you are using an RC oscillator as the feedback network and an opamp as a amplifying device, then by Barkhausens criteria the gain * feedback = 1 for sustained oscilations. Initially assuming that only the resistor noise is available at the input of the OPAMP, the OPAMP has sufficient gain to amplify this noise. i.e the o/p of opamp tries to move towards saturation ( just by the noise). Here the point is that the necessity os sinusoidal component is not important, and at any point of time a noise signal can be defined as a sinusoidal component by determining the fourier component of noise.