If normal helicopters make so much noise that can be heard kilometers and kilometers away, on a city full of other noises, like cars, buses, people etc.

If the Apache helicopters are much bigger (and I guess much louder) and they fly on (many times) very quiet places (deserts, and there are no shots being fired) and everyone is on high alert.

How can they keep on flying and flying, circling terrorists/enemies without no one ever hearing them, as I have seen on many footages? How high can they fly to not be heard and from how far can they engage?