Noise pollution is the second major component constituting Environment. Noise pollution is emitted mainly ny streets congested with traffic, airports, windmills, Sports Leisure buildings and some of the resource boosters. The fact is that noise pollution is often more difficult to keep in check, because its sources are spread everywhere in the city, rather than contained in a distant industrial district.
Sports facilities such as Bowling alleys and Tennis courts all emit noise, and they need to be relatively close to your residencial areas (for their Ares of Effect to apply), so you need to counter their effect with Landmarks and parks. However, they might harm more Quality of Life-sensitive industry and commerce, so keep them away from those.
You can reduce traffic noise of course by reducing the amount of cars that pass by. This you do by spreading the cars on more road, or by forcing them to take other routes (by using one-way roads for example). Busy Expressways are the worst offenders because one cannot build buildings along Highways. Wooded Park Areas help greatly to let the people worry less about noise pollution from traffic, so make sure you got them near your citizens, especially Elites and Executives. The other noise polluters you should just keep far away from your citizens, so place the windmills and airports somewhere in a corner of your map.
Like Air pollution, there are not many ways to reduce the amount of pollution itself, so rather than doing that you should seek to counter it, first by keeping sensitive buildings like High Tech Industry and Offices away, and second by building Landmarks.