To unlock the whistle in Animal Crossing: City Folk, you need to follow these steps:

1. Ride the Bus with Kapp’n:
– Ensure you are in the game and have access to the bus that Kapp’n drives. This is typically when you are traveling to the city from your town.

2. Wait for Kapp’n to Start Whistling:
– If you leave the game idle long enough while riding the bus, Kapp’n will begin to whistle Totaka’s Song.

3. No Additional Actions Required:
– There are no specific actions you need to take other than waiting for Kapp’n to start whistling. This is a natural part of the game’s progression and does not require any additional items or tasks.

By following these steps, you will hear Kapp’n whistle Totaka’s Song, which is a unique and recognizable tune in the Animal Crossing series.