http://www.zdnet.com/article/silent-rotor-blades-could-lead-to-true-stealth-helicopters/
According to this, the noise comes from blade encountering vortexes shed off the preceding blade.
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.Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster
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.Impossible is just something we haven't done yet. -- Langley Outdoors Academy
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