obnoxious minority

A small-medium group in a larger coalition of people (Fandom, Political Party, etc) who do not represent the values or desires of the coalition. However they make so much noise and are so loudly outspoken that they come to represent the coalition as a whole. Often regarded by their peers as "annoying loudmouths".

The opposite of the Silent Majority.

"The Westboro Baptist Church really is the Obnoxious Minority of the Christian faith"