When a body is not adequately supported, it can translate or rotate as a whole without deformation. A body without any restraints has six rigid body modes; 3 translations and 3 rotations. The software finds rigid body modes in addition to the requested number of flexible modes. The frequency of a rigid body mode is zero which means an infinity period.
Video: Sample Rigid Body Modes
You must restrain the model properly for linear dynamic studies.
In cases of bonding contact between sheet metal parts with other surfaces (solids or shells), the bonding algorithm can enforce over-stiffening due to the gap between the mid-surface mesh and the contact surface geometry. The over-stiffening of the bonding contact alters the calculation of the rigid body modes. The program will calculate all rigid modes (with frequency values close to 0), if you eliminate the gap by offsetting the mesh of the sheet metal parts from the mid-surfaces to the contact surface geometry.