rational explanations anyone?

Earlier this afternoon, I heard a great commotion at the front door, as if someone was trying to enter. My wife heard it too. The dogs started barking and continued to bark for several minutes. I went to check, and no one was near my door, no driving cars, nothing. A minute later I heard the upstairs fan go on, with one of the blades on the floor. Rational explanations anyone. No "its my imagination" type nonsense either. Could a small earthquake or something cause this, without affecting anything else? I wouldn't like to think a visitor is here, or if it was, hoping that it moved on directly. Nothing else has happened since this.