Hi Morteza,

When you dont have any constrain in the structure, you will find rigid body modes which are close to zero in modal and harmonic response analysis. Constrains are not necessary. Also, you can think some structures have free free boundary conditions such as space ship, plane etc. In this type of structures there is no boundary conditions. On the other hand, vibration tests generally are tried to execute under free free boundary conditions. In this type of test, you can see also rigid body modes in FRF functions low frequencies. Moreover, test fixation elements such as elastic bands, springs etc. are used. Stiffness of these type of fixations elements effects the rigid body modes but they provide free free boundary conditions for the structure. 

Best Regards