Whismur

Normally, Whismur's voice is very quiet—it is barely audible even if one is paying close attention. However, if this Pokémon senses danger, it starts crying at an earsplitting volume.

When Whismur cries, the sound of its own voice startles it, making the Pokémon cry even louder. It cries until it’s exhausted, then it falls asleep.