Dear Simon,
As described in this forum topic: Learizing Baseline 5MW Wind Turbine with FAST, rigid-body modes (i.e. modes without stiffness) show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping (i.e., eigenvalues with real values only). That is, each rigid-body mode will introduce an additional mode beyond the number of enabled DOFs and the damping is unphysical. When aerodynamics are enabled, perhaps there is a small amount of aerodynamic-induced stiffness that causes the rigid-body generator mode to have a slightly positive frequency.
Best regards,