My cat makes weird noises every night, sounds like shes saying "Hello?"?

Every night, when every one is asleep, my female cat (shes spayed) meows, and it sounds like shes saying Hello, in two syllables, no joke! It really creeps me out. Shes had a normal cat life, and as far as i know, she has no health problems. She also sometimes does it during the evening, when me and my family are upstairs watching a movie, and she is downstairs. Can someone tell me what it means? Im really curious....