cal instrument they used in the old sci-fi movies when the monster was about?

[bruceemasters]

Back in the'50s they used it whenever a monster as emperilling por humans. It was sort of a whistle.

 

[Will1]

Probably an organ or later a therimin. Or maybe even a steel guitar.

 

[PanamaJoe]

Will mentions the theremins, a musical instrument that produces that hollow "Ooooooh" sound. In "Hannibal" Dr. Lecter is seen playing one. In some films and TV, though, if you listen, it's someone singing the notes, ala the original theme song for "Star Trek." It's a woman singing, and it may be Majel Barret, wife of producer/creator Gene Roddenberry.