8 years 737 up to '97, since then continuously on the 747-400. I found the 737 (except the -200) vastly quieter than the 747 due to the lower speed- the 737-400 very relaxed and quiet. The 747 has a pronounced roar which I have always been told is due to the sharp fuselage curves around the roof of the Flight Deck. I expect in addition, after 18000 hours of sitting on top of jet engines, we all lose different parts of our hearing (permanently), so perhaps I am happily sitting there in blithe ignorance unable to hear a particular tight frequency band which is where all the roar is? I know having seen my audiogram trace that there is an astonishing degredation of hearing over the years.