Question

what do we call the part of a song that you sing, whistle, or hum?

Explanation:

In music (a subfield related to Arts, specifically in the study of musical structure), the part of a song that is typically sung, whistled, or hummed is the melody. The melody is the main musical line that is memorable and is what people often recall and reproduce when singing, whistling, or humming a song. It consists of a sequence of musical notes that form a recognizable tune.

Answer:

The part of a song that you sing, whistle, or hum is called the "melody".