For people working on their spiritual growth, there may come times when they hear a high pitch sound. The sound is their personal frequency changing. It usually signifies that they have made a decision or something came to action that is raising their frequency and taking them to another plane.
Twin flames, or others who are spiritually working together, sometimes hear this at the same time because they are taking spiritual steps together. If one person hears it and the other does not, it may be for some aspect of life that just pertains to one of them.