I heard/saw the hems go pretty much overhead (2/3nm) from royal london this morning and the noise didn't seem too bad at all (to a pedestrian on their way to work) - there's a bit of a screetch as its flying away from you. Interesting reference below to twin squirrel vs 902 noise:
http://www.southyorks.police.uk/depa...ss/helicopter/
Going back in time, i was walking around whitechapel in '99 when the previous incarnation of hems landed (dauphin?) and the noise from that on approach was literally scary. There's something about London streets (e.g. flat fronted terraces on both sides) that can conspire to create a real cacophony.