It does not have anything to do with mecanical stress, quite the opposite, at this moment the torque is very very Low, it does have to do with aerodinamics and nothing to be worried.
Better late than never: it's the sound an advancing blade makes when it hits the turbulent air left by the preceding rotor blade. That's why you can create this sound at will by descending while slowing down. And because we're good boys we can avoid it for our friendly neighbour's sake by performing a very rapid descent at higher airspeed first and then slow down at your target altitude afterwards (if the operation permits it)
