Just go and try what I suggested. If you do it smoothly you will encounter transverse flow (inflow roll) towards the advancing side with NO vibration. It is due to the difference in the inflow angles at the front compared to the rear of the rotor disc.
You know the rotor produces vortices at the tips right? Those vortices are what cause the vibration as you transition to and from the hover because the rotor has to battle through them into clear air - that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift. The vibration marks the boundary of ETL and that is why, once you are through it - it doesn't keep vibrating.
If transverse flow was the reason for the vibration, you WOULD feel that throughout the flight envelope whenever you tilt the disc and create an inequailty of lift.
Don't confuse flapback (blowback) with transverse flow (inflow roll).