When walking over a hole covered with a trapdoor, and underneath the trapdoor there is  ladder, the ladder sound effect will play as if you are climbing a ladder even though you aren't.

Instructions to replicate - 

1. Find a flat spot. 
2. Dig a 2-block hole.
3. Place ladders in the hole.
4. Cover the hole with a trapdoor.
5. Walk over the trapdoor, you should hear the ladder sound effect.