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powersloshin said..
Most probably it's the fin, just lightly sand the trailing edge and it will fix it . You need to do a couple of passes on the trailing edge, but perpendicular to the fin, you don't want to sharpen the edge, but make it a bit more 'square'
Actually a bit more dull, "square" can make it worse.
The fin can whistle also if it's crooked, twisted (if a molded fin), the box (for a single fin) is not pointed straight ahead or slanted, or the fin was poorly foiled to begin with.
But I've never considered it a whistle, more of a buzz, surfboards can have the same problem, been there.