Wikipedia
Harpua
"Harpua" is a Phish song that was a staple of their live concerts, but has now become a rarity and has only been performed live. Though it follows the same basic structure in every performance (see The Divided Sky and You Enjoy Myself for other examples of songs in which variation occurs within a fixed structure), Trey Anastasio narrates a different story at every concert.
The song's introduction is an a cappella "oom pah pah" melody sung by all members of the band.