In a first analysis of the low-frequency fluctuations observed by Cluster during a crossing of the plasma sheet <Louarn et al., 2004>, we identify oscillations that likely correspond to MHD eigenmodes. They are intense (10 nT), have a rather short period (20--25 s), have a wavelength of ~5 Re, and are observed during an active period that follows a substorm onset. We show here that they combine both sausage and kink modes propagating in the x direction with a common spectral peak at ~0.04 Hz, the sausage oscillations presenting also a secondary peak at 0.13 Hz. Using a complete theory of the linear MHD response of the plasma sheet, we reconstruct the sheet oscillations starting from given external perturbations. This method leads to quantitative predictions concerning the magnetic and pressure fluctuations. The existence of natural resonant frequencies of the sheet can indeed be foreseen and the relative weight of the kink-like and sausage-like modes can also be estimated. We show that the kink mode (0.04 Hz) and the high-frequency sausage mode (0.14 Hz) are perfectly compatible with the MHD model of Harris sheet oscillations, with wavelength of ~6 Re and ~3 Re, respectively. The corresponding amplitude of the pressure and magnetic fluctuations also appears to be remarkably consistent with the theoretical predictions. Conversely, the low-frequency sausage mode (0.04 Hz) is hardly explained by the MHD model. We propose that it could be a nonideal MHD or a kinetic mode, as the tearing mode.