why did the cops knock and holler we are cops and we are coming in when they had a special issue 'no knock warrant'?

Answer 1

Because in America many people have guns and they have the right to defend themselves in their homes and if the police do not announce who they are how is the homeowner supposed to know this is a legal entry and not exercise their right to shoot an invader to their home in self defence.

Answer 2

The warrant did not prohibit them from knocking, it just gave them the option of not knocking if they felt it was a better. Many times these decisions are made at the last moment. 

Answer 3

They did not announce themselves at all.

Answer 4

cause they wanted to check stuff out