Disappeared into one of the cooling ducts.
4 seconds sound clip from the Alien (1979) movie soundboard.
You can hear this line at 01:08:22 in the Blu-ray version of the movie.
Quote context
[...]
- Hey, hey. I'm not gonna hurt you. Come on.
- Whatever it was, it was... it was big and...
- You're sure it took him into the air shaft?
- Disappeared into one of the cooling ducts.
- No question, it's... it's using the air ducts to move around.
- No. I mean, I don't think so.
- Well, this air shaft may work to our advantage.
[...]