When performing operational modal analysis the dynamic loading is unknown, however, once the modal properties of the structure have been estimated, the transfer matrix can be obtained, and the loading can be estimated by inverse filtering. In this paper loads in frequency domain are estimated by analysis of simulated responses of a 4 DOF system, for which the exact modal parameters are known. This estimation approach entails modal identification of the natural eigenfrequencies, mode shapes and damping ratios by the frequency domain decomposition technique. Scaled mode shapes are determined by use of the mass change method. The problem of inverting the often singular or nearly singular transfer function matrix is solved by the singular value decomposition technique using a limited number of singular values. The dependence of the eigenfrequencies on the accuracy of the scaling factors is investigated and the errors on the estimated loads are determined.