For one thing, A330's are more fuel efficient and (IIRC) have a lot more cargo uplift capacity than 767's. On the other hand, 777's beat A340's in those categories. So from an airline's point of view, A330 > 767 but 777 > A340. From the passenger's point of view, I think any difference in aircraft-generated noise is swamped by differences in passenger-generated noise -- crying babies, rowdy jr. high school students on a class trip, etc. I bet if you pay attention to that factor you could correlate apparently quiet cabins with a light pax load, far more reliably than you could correlate it with aircraft type.