I agree that Hanoi sounds really pretty and has beautiful meaning

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Ha = river
Noi = inside, within
Hanoi= city within river
It indicates that the city is found on a bent of a river (the Red river). The city is also very close to the Black river (sông Đà), and the Clear river (sông Lô).
Past names of Hanoi include: Thăng Long (ascending dragon), Đông Kinh (eastern capital ~ japanese city), Đông Quan (eastern gate), etc
Most Vietnamese cities have meanings though.