So I heard Crow whistling this one time and I thought "Oh, that's cool! He's whistling the theme to Shadowkeep!"
But apparently he's actually whistling Savathûn's Song? Are the main theme of Shadowkeep and the Song the same?
[spoiler]I don't have the doggoned expansion so if that was part of the story, I didn't have a chance to see it.[/spoiler]
Otherwise, my thought would be is that Bungie inserted a cool little Easter egg and a nod to the last expansion.