Most operational modal analysis algorithms are based on the assumption of white noise excitation, which has some application limitations. A new method utilizing transmissibility functions is presented without the white noise hypothesis. Rational functions (RF) are constructed by combining transmissibility measurements under two different loading conditions, from which modal frequency, damping ratio and modal shapes are obtained by the Forsythe orthogonal polynomial fitting as rational fractions in the frequency domain. Finally, a simulation example of a wing model and an experiment of a cantilever beam are used to validate the method. This technique achieves good results, even if the excitation is not white noise.