The ex-DEA agent from the "Don't Get Busted" videos says to never open your door for police, not even to step out and close the door behind you. Police can claim they smell bud and have reasonable suspicion to search your home. He says to show them your hands through a window and if they have a real warrant they will kick your door in. He says as an ex-fed he would lie to people about having a warrant to get them to open the door, and then claim to smell bud to have probable cause to get a real warrant. You are under no legal obligation to open your door for any police officer. It is perfectly legal to ignore an officer knocking at your door. If they had a real warrant they wouldn't be knocking and talking, they would be ramming your door down.