DC,
I'm not sure whether the actual volume of the noise is the issue, so much as the nature of the noise. Eurocopter make a big thing in their blurb about the measured PNdB being lower than the competition; however, I suspect the thing that makes people hear the 135 before the 902 is the "harsher" (subjectively assessed) note. We even had calls on my last unit when the 135 from an adjacent unit was overflying our area. Because to Joe Public one blue-and-yellow helicopter looks like another, some folk were concerned that there might have been something wrong with our a/c as it went past.