In the article, we have proposed to use microwave-fluctuation meters resistant to external intense electromagnetic noise in order to measure the low-frequency (LF) noise of microwave transistors working under these conditions. The transistor located on the board is excited by a low-noise microwave generator, the oscillation amplitude of which, being modulated by the LF noise of the transistor, is measured by a microwave spectrum analyzer. The proposed method was tested on GaN/AlGaN heterotransistors, in whose channels the electron density was formed by spontaneous and piezoelectric polarization. In addition to experimental testing, a theoretical justification for the method is presented. We obtained conditions in which the normalized spectra of oscillation amplitude fluctuations were similar to the normalized LF noise of the transistor current.