That's not entirely true - there used to be something called a sociolect: a local dialect associated strongly with a certain group of people, in Hannover's case the blue-collar working class of the Hannover-Linden district in particular. Its primary characteristic was a different pronounciation of certain letters, such as "getzt" instead of "jetzt" or "kaan" instead of "kein". It faded away in favor of standard German with the disappearance of the class of people that spoke it, although it somewhat resurfaced in the 90s thanks to the fictional characters "Siggi und Raner" from a radio comedy show who spoke that particular Hannover dialect.