This is one of their ventilator shafts. You can feel the warm air rising.
6 seconds sound clip from the Forbidden Planet (1956) movie soundboard.
You can hear this line at 01:01:16 in the Blu-ray version of the movie.
Quote context
[...]
- Listen. Circuits opening and closing.
- This is one of their ventilator shafts. You can feel the warm air rising.
- Look down here. Look down, gentlemen. Are you afraid?
- Seventy-eight hundred levels...
- And 400 other shafts like this one.
[...]