The 206 L has the nodal beams that dephase & buckle the tail boom as the rotor Rpm decays on the landing phase of autorotation, The Bell 205 & UH 1 will do a good whirl when the simplex water tank fills with water and the pilots start to look like a load of laundry in an outa ballanced washing machine ( It starts small & grows with the change in C/G and load.)
Bad Txsn dampers too will add to the excitment, Or bad sprag mounts in the Hiller & Bell 47, or a bad isolation mount in the 206 Bs
Did a lot of vibration tests, funny what spinning things do in a helicopter.