Police ARE NOT your "friend". Their is much angst about inner-city blacks not cooperating with police. Frankly, I don't blame them. Police have turned into an "army of occupation" in many areas. Militarization of police forces has fostered an "us vs. them" attitude in which the cops are seen as the "good guys" that get away with anything, even murder.
No-nock warrantless (or warrants issued on flimsy or no "evidence") raids do nothing to endear police to citizens. EVEN THE NAZIS KNOCKED ON THE DOOR BEFORE GAINING ENTRY. I repeat: police ARE NOT your friend. Be very wary when dealing with them.