On my one Chinook ride, we had the rear door open thoughout and it wasn't too bad, mind you everybody was wearing headsets which would have muffled the noise a bit.
A few years ago (about 2016 I think) I was out in my back garden 'slugging' (unfortunately we've never been blessed with a resident hedgehog) at 4.25am and on H3, about half a mile north of me and about 2 miles east of Bagshot Mast a pair of wokkas ambled along westbound at about 1000ft and I was amazed how little noise they made in the otherwise dead calm of the dawn; somebody on here said it was a new design of rotor blade.