That happened to me last year, I got up early an heard a rapid tapping (I don't have hot water heat) and could not figure out what it was. Thouoght someone was knocking on my door. It was dark outside at this point. Sooo when daylight finally broke I saw out the window a bluejay of all things tapping on our drain pipe. He came back every morning for several days. Don't know what he thought he was getting LOL