Cats hear the calling over vast distances...male and female cats are calling to find each other...last year a feral female cat was howling around the neighborhood...it's such an agonizing sound...
I've wittnessed that Nellie and some neighbor cats promptly react to this...as being spayed and neutered they have no sexual interest, but the cat was roaming THEIR territories in an unacceptable loud manner...
So I guess Barbara is right here, it's a territory thing...