Strong Updraft
Flying quite a bit in Mountainous area's, I regulary get updrafts that are more powerful than the autorotative capacity of the heli.
As TFS described it - not hovering but level- I assume he lowered collective quite a bit to stay level, so entered in full autorot, which will not pass via a VRS-area, because reversal is fast.
Even a few weeks ago I nearly had an overspeed because of trying to maintain altitude and lowering the collective to far. A complicating factor is that the wind streams are turbulent, so with low collective you can still be hit by a stronger updraft, which at this points requires you to pull collective and start climbing to avoid overspeed. It helps to know that there is no solid deck right above you, or strict altitude requirements at this stage....
So imho the sound could have been overspeed
d3