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.
Savathuns Song is viral chant and can never be unheard. We do not know what it can do but people think might be mind control. If it is, that is suuuuper bad. Because Lord Shaxx has heard it, Saint has heard it, Crow has heard it, Eris has heard it, Drifter has heard it, and WE have heard it. Two legendary titans and 3 important characters have the chance (if the destiny's community speculations are true) to be mind controled which is super awful. Even we have the chance to be mind control and probably a bunch of other guardians too!