aa777888 - another thing for you to try next time you go flying.
Make a normal constant angle, speed reducing approach until you get to the burble/vibration and then hold the speed steady.
Once happy in that condition, note the power required to maintain your descent angle.
Now reduce speed slowly until the vibration goes away and stabilise again at that new speed.
Note the power required to maintain your descent angle.
I expect two things to be obvious, 1. you need more power to maintain the angle as you have lost ETL and 2. you are not having to make roll corrections with cyclic during the journey through the burble/vibration because it isn't transverse flow effect.
Transverse flow (inflow roll) is noticeable on a transition from the hover because of the difference in inflow angles between front and rear of the disc - I hope we can all agree with that - but is barely noticeable on the approach, yet the vibration occurs clearly in both transitions - discuss
