They can knock on my door all they want - I won't answer. If I don't know who you are, I don't open my door. If they have a warrant, nothing I can do is going to stop them from executing that warrant. So, if they are not kicking in my front door, I can assume there is no warrant, and if there is no warrant, I don't engage.

Never answer questions when approached by law enforcement - any questions. Do not engage them by answering even what seems like the most simple of questions - like your name, etc. This is how they trap people - especially the Feds. Anything you say can and will be twisted to use against you. Don't play their game. Opt out. Even with a warrant, do not answer questions - that is your right. When asked a question, simply reply, "I do not answer questions." Ask any defense lawyer and they will tell you that their client's worst enemy is the client themselves because of what they say to law enforcement. We have a Constitutional right to remain silent - exercise it.