In The Prish, level 4, when you come into the Diner with the pool tables up stairs.

On the first floor is a piano, in the corner on the stage, I think its a stage.

Go up to the piano and press X many times and its wierd, I will not tell the rest, you must find out yourself.

PS: don't scream when you hit X. Another thing to it's better to play online when you do it, because you can see the hand of the player press the piano.