We report the observation of the thermal equilibrium strain fluctuation spectrum in a transducer using a fiber interferometer. The thermal noise power spectrum in the range of 20–30 kHz of the metallic glass cylinder in this experiment is determined by the dynamic elastic susceptibility and has peak value corresponding to approximately 0.8 $μ$rad/Hz at 25.5 kHz. The data are analyzed by using the fluctuation–dissipation theorem and a coupled-mode model for dynamic response in metallic glass.