And now consider your previous comment about how the vibration is transverse flow as you reduce speed on the approach.
You perhaps will agree that in the transition to forward flight, the disc is tilted forward and the difference in inflow angle (small angle at the front and large at the back) is the cause of the transverse flow effect (I say it rolls, you say it vibrates).
If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).
The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.