No-knock raids don't "knock" (ie, announce their presence and wait for you to open the door). But they do usually announce their presence:
https://en.wikipedia.org/wiki/No-knock_warrant
"In most cases, law enforcement will identify themselves just before they forcefully enter the property."
The justification is rapid entry to either prevent suspects from destroying evidence or to prevent them from mounting an organized resistance. But they still generally announce their presence. Doing so doesn't compromise this goal, and can reduce the odds of armed resistance (drug dealers who would shoot rival gangs might not risk attacking police) and also provides better legal justification for any shooting by police or prosecution of anyone who shoots at police.
But it's not foolproof. In addition to the possibility of people being asleep or otherwise unaware for the initial announcement, there's also the problem that non-police home invaders can claim to be police.
This has happened before, so the announcement alone cannot constitute proof that they are police. And if the police get the wrong address and target a home with no criminal activity, the occupant might believe a home invasion is more likely.