Major source of noice pollution within urban areas are cars, lorries, trains, aircraft taking off and landing, factories, large groups of people, and outer noice from bars, nightclubs and residential areas (such as house parties.)
The amount of noise these sources leak depends on the time of day, location within the Urban area and day of the week.
This tends to be worse in areas that haven't been well planned out e.g. places with industrial and residential ares close to each other.