It was... screaming. Some kind of screaming, like a child's voice.
10 seconds sound clip from the The Silence of the Lambs movie soundboard.
You can hear this line at 01:10:39 in the Blu-ray version of the movie.
Quote context
[...]
- Then something woke you, didn't it? Was it a dream? What was it?
- It was... screaming. Some kind of screaming, like a child's voice.
- I crept up into the barn house.
- I was so scared to look inside, but I had to.
[...]