What tune did the Man in the Yellow Suit hear near the Foster cottage that convinced him that he had finally stumbled onto the "family that didn't grow older"?

The pretty little music box, adorned with painted flowers, belongs to Mae Tuck. When Winnie first hears the strange tune emanating from the forest, her grandmother refers to it as "elf music" as there's something wonderfully weird and other-worldly about it. The music box symbolizes the Tucks' presence in the world, even when no one can actually see them.
Many years before, the Man in the Yellow Suit had heard the tune; it was taught to him by his grandmother. Try as he might, he just couldn't get it out of his head; the melody even appeared in his dreams. So he set out to track down the immortal family of his grandmother's stories, to see if they were real or just figures of legend. When he approaches the old Foster home and starts hearing the music box play, he knows that the family of immortals he'd heard about for so long is real and that he's finally found them.