Which song is this song referencing?
 

I cannot post a URL yet (new user), the song is "Love and Sin" by U.D.O. On Youtube their channel is UDOtubetv. The album it belongs to was a joined production of the heavy-metal band UDO and the Bundeswehr Musikkorps (German army orchestra).

At the beginning (20s-35s song time, played on recorder) and at the very end (and as theme for some sung parts in the middle) this song embeds/references another song/melody. One that I know and that I think should be somewhat well-known- but I don't remember at all what it is right now.

Does anybody know that melody?

It sounds vaguely familiar - maybe. Might even be a passage from some classical music piece?