Who Can It Be Knocking At My Door?

Maybe it's the Federal Communications Commission, but the knocking is only a courtesy. They think they can just come in and help themselves (via Volokh):

You may not know it, but if you have a wireless router, a cordless phone, remote car-door opener, baby monitor or cellphone in your house, the FCC claims the right to enter your home without a warrant at any time of the day or night in order to inspect it.

That’s the upshot of the rules the agency has followed for years to monitor licensed television and radio stations, and to crack down on pirate radio broadcasters. And the commission maintains the same policy applies to any licensed or unlicensed radio-frequency device.

It's not at all certain that this would pass Fourth Amendment muster (Conspirator Orin Kerr opines in the article that it would not) and it's never really been tested in court. Still, I wouldn't want to be the one to go through the trouble of proving it.