Vibrations (1968)

Barbara, a frustrated writer who is also sexually frustrated, seeks to recharge her life by moving into a Manhattan apartment, where she earns money typing up manuscripts for other writers. Next door, a sexy young lady plays a variety of erotic games revolving around her hypnotically humming vibrator. When Julie, Barbara's hot-to-trot sister shows up, she reluctantly allows her to stay, even though memories of incestuous girlhood explorations between them cause her obvious distress. Julie soon seduces one of her sister's male clients (whose advances Barbara had earlier spurned) and gets involved with the girl next door, her entourage and her toys. Barbara soon finds that her long walks in the park no longer provide adequate resistance to her unfulfilled passions and curiosity about the sounds emanating from behind the wall.