It is an homage to Das Rheingold, one of the Ring operas, and a direct reference to the legendary gold in the Rhine River, of which the Ring of the Nibelungs is fashioned.
ɪɾ ɪz æn hˈɑːmɪdʒ tə dˈæs ɹˈeɪŋɡoʊld, wˈʌn ʌvðə ɹˈɪŋ ˈɑːpɚɹəz, ænd ɐ dᵻɹˈɛkt ɹˈɛfɹəns tə ðə lˈɛdʒəndɚɹi ɡˈoʊld ɪnðə ɹˈaɪn ɹˈɪvɚ, ʌvwˈɪtʃ ðə ɹˈɪŋ ʌvðə nˈaɪblʌŋz ɪz fˈæʃənd.