Vibration when going out of ETL
Vibrations come from the main rotor blades interfering with vortices of preceding blades (cfr Nicks references). At slower speeds these vortices are not shed backwards. Gordon Leismans book 'Principles of helicopter aerodynamics', shows several nice drawings about this at different speeds (Section Rotor Wakes and Tip Vortices, p 426 onwards)
The difference in behaviour/strenght of this vibrations between different types and even different helos of the same type, are also influenced by different resonant behaviours of the main rotors, this can also depend on the balance of the blades.
Delta3