The Alleluia itself is the "refrain" of the hymn tune GELOBT SEI GOTT, most often seen as the setting for the Easter hymn "Good Christian Men, Rejoice and Sing" (not to be confused with "Good Christian Men, Rejoice," the Christmas carol) or the Christ the King hymn "Christ Is the King, O Friends, Rejoice." If you're referring to the introduction, I'm not sure, but it could very well simply be the organist's improvisation.
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