What contributes most to noise levels in cities?

I'd say the biggest contributor to noise in cities is traffic-cars, buses, and motorcycles. The main reason is that urban areas like Shanghai have such high vehicle density, especially during rush hours. For example, when I'm commuting to work, the honking and engine sounds are almost constant, especially near intersections. That said, construction sites also add a lot of noise, especially in rapidly developing areas. They're unavoidable, but they definitely make cities louder than they need to be.