Look at the Modal analysis. Plot the First Mode and play the animation. Look at the point on the model that has the maximum deformation. Look at the direction that point is moving. Is it approximately moving along a Global Coordinate direction? If not, create a coordinate system that has an axis parallel to the path that the maximum point is moving. Plot Directional Deformation of that point and select the direction which aligns with the path that point move along in the First mode animation. Now you will have a plot that crosses zero and oscillates like a sine wave.
With 4% damping and a 90 Hz first mode, here is the displacement response of a mass-spring-damper system excited by a 100 cm/sec initial velocity.
If you don't get this kind of decay, there is something wrong with your model.