I live in India, and near an airbase so there are a moderate amount of helicopters flying overhead. My friend claims that he can distinguish between a civilian helicopter or a military one based on the engine noise. He says that military helicopters have a higher pitched whine in their noise that is absent from civilian ones. I have heard it myself but I have no idea if it's perception after he told me or reality. Is there any truth to this? Keeping in mind that most military choppers are Russian/Soviet designs in India while a lot of civilian choppers are Bells
I think your friend is playing a joke on you. I have worked with helicopters since the late 1970's and there is no way to distinguish military from civilian simply by engine noise. You may be able to distinguish between models of helicopters, and in so doing identify a model used by the military versus a civilian one, but that's about it.