A novel measurement system with thermal noise limited sensitivity has been developed for studying the fluctuations in passive microwave components. The developed system is capable of measuring the noise in the quietest microwave components in real time. The noise floor of a 9 GHz measurement system was measured to be -193 dB/Hz at offset frequencies above I kHz. We present the measurements of the phase and amplitude noise in precision voltage controlled phase shifters and attenuators, along with the first reliable experimental evidences regarding the intrinsic phase noise in microwave isolators. Also we discuss the possible applications of the developed noise measurement system for noise reduction in the microwave amplifiers and oscillators.