Dear Riccardo,
For information on the FAST theory basis, please see the following forum topic: Coupled blade modes in FAST. That said, much of that doesn’t apply when all DOFs are disabled except the drivetrain and generator DOF.
Are you using steady uniform wind without yaw error? Even so, the NREL 5-MW turbine has some rotor tilt, which may lead to oscillations in aerodynamic loads. I’d probably have to know more about your simulation setup and results to comment more.
Best regards,