I don't know, it was just some kind of big outline in the disintegrator beams.
8 seconds sound clip from the Forbidden Planet (1956) movie soundboard.
You can hear this line at 01:23:10 in the Blu-ray version of the movie.
Quote context
[...]
- Quiet.
- Robby, emergency cancellation Archimedes.
- We were attacked. Three more men dead, including Jerry Farman.
- I don't know, it was just some kind of big outline in the disintegrator beams.
- No.
- Well, anyway, we fought it, and we lost. I figure it'll be back.
[...]