When Frodo was in the Barrow, He sang Bombadil's rhyme. This rhyme summoned Bombadil to help them. How was Bombadil able to come within a moments notice? If Bombadil was at his house wouldn't it take time for him to get to the Barrow? Was he following Frodo and company?
There was a sudden deep silence, in which Frodo could hear his heart beating. After a long slow moment he heard plain, but far away, as if it was coming down through the ground or through thick walls, an answering voice singing.
How was Tom able to respond after only a moment?