What song does Data whistle in the first episode of generation Star Trek?

You're likely thinking of the first episode of Star Trek: The Next Generation, "Encounter at Farpoint," where Data whistles the tune of "Pop Goes the Weasel".

While he doesn't whistle the entire song, it's a very recognizable melody, and Data's rendition is somewhat robotic and endearing. It's a moment that highlights his artificial nature and how he's trying to learn and understand human emotions.