It is demonstrated that the noise characteristics of the injectors of linear electron accelerators (the spectral densities of transverse deviations and of bunch velocities) can be measured with volumetric resonators independently of the critical current. These quantities are measured at the injector output near the frequency at which instability develops in LUE-2 GeV accelerator. Their dependences on the current in the range of 0. 1 - 1 A with different methods of its regulation are obtained.