Both periodic and chaotic oscillations are studied numerically in the distributed dynamic system with interacting of backward linear and nonlinear waves at the presence of additive noise in nonlinear wave. Studied distributed system is the simplest model of the electronic backward wave oscillators with non-inertial bunching mechanisms. It is shown, that excitation of а lot оf space modes is typical for such systems. It is found, that spectrum оf chaotic oscillation near their excitation threshold is very non-uniform. In this case an external noise is «superimposed» on self-sustained oscillations. In the mode of chaotic oscillations at high-current beam under the external noise influence, it may be both increase of chaos and regulating of generated signal form, exhibiting the properties inherent in effect of stochastic resonance.