I've always heard blade vortex interaction is the main source of noise.
The four main noise types are: BVI, HSI (High speed impulsive) thickness noise and of course tail rotor noise. Of these four, which is the biggest contributor as a percentage? Or is it even scientific to rank order them?
Listen to the sound of the H160 compared to the AS365 without the BVI reducing swept tips (Blue edge), the difference is clearly noticeable.
IBC can also reduce HSI noise, is the added complexity just not worth it? The video below of the BO-105 equipped with IBC clearly demonstrates it is highly effective.