With careful planning and the right offshore wind/ topography, we took a Chinook full of interesting people to a bay N of Saxa Vord and were told by the exercise DS that no-one heard us until we made our first turn after departure some 4 miles offshore. Conversely, if you really want to be noticed - perhaps, say, by the participants in a Mount Pleasant dining-in night who've 'forgotten' to invite anyone from 78 - descend at 4-600fpm/ 100kt aiming at a point just short of the target then pull up into a 30deg bank climbing turn approx 400m short so that you don't overfly any buildings. Lifts the cutlery off the table, apparently.
Never believed the Eurocopter publicity about their 'quiet' helis - the 135 was always more audible than the NOTAR MD902, particularly around the home base(s). I'm pretty sure that the latter's 5-blade head producing vortices that didn't interact with any kind of external fan was the reason.