Oscillations of the current sheet observed by the Cluster spacecraft are investigated, using the special ability of Cluster to separate between spatial and temporal variations. We find that before substorm onset, a thin current sheet moves with a velocity of 10 km/s mainly in the vertical direction; after substorm onset the current sheet thickens and moves with greater velocity, ~25 km/s. Oscillations of the current sheet observed before and after substorm onset can well be interpreted and modeled as a driven magnetoacoustic, with different values for the (Harris) current sheet half thickness and the current sheet velocity before and after substorm onset. The damping rate of the wave can be shown to be related to the thickening rate of the current sheet.