Manos: The Hands of Fate

While on a road trip near El Paso, Texas, Michael, Margaret, their young daughter Debbie, and their dog, Peppy, search for the "Valley Lodge". Michael and his family finally reach a house which is tended by the bizarre, satyr-like Torgo, who takes care of the house "while the Master is away". Michael and Margaret ask Torgo for directions to the Valley Lodge; Torgo denies having knowledge of such a place. With this information, Michael asks Torgo to let him and his family stay the night, despite objections from both Torgo and Margaret.
Inside the home, the family sees a disturbing painting of a dark, malevolent-looking man and a black dog with glowing eyes; the man it depicts is the Master. Margaret becomes frightened upon hearing an ominous howl; Michael investigates, retrieving a flashlight and revolver from his car, and later finds Peppy lying dead on the ground. Torgo reveals his attraction to Margaret and tells her that, although The Master wants her to become his bride, he intends to keep her for himself. Margaret threatens to tell Michael of Torgo's advances, but Torgo convinces her not to say anything to her husband by promising to protect her. Michael returns, unable to start the car. With the revelation that there is no phone in the house, the family reluctantly decides to stay the night.
Michael and Margaret stumble upon "The Master" and several women dressed in translucent nightgowns and later revealed to be his wives. They are all asleep. Torgo uses a stick to knock out Michael, and then ties Michael to a pole, after dragging him to it, and The Master suddenly comes to life. His wives also awaken, and a short argument over the fate of the family ensues. The Master decides he must sacrifice Torgo and his first wife to the film's mysterious deity and namesake, "Manos." When The Master leaves, his wives engage in further argument that soon degenerates into a fight, and the women wrestle in the sand.
Torgo succumbs to what appears to be a hypnotic spell by The Master. The Master stops the fight, and has his first wife tied to a pole to be sacrificed. Torgo is laid on a stone bed, where he is attacked by The Master's other wives, but this in itself does not prove fatal. Evoking some mysterious power, The Master severs and horribly burns Torgo's left hand. Torgo runs off into the darkness, waving the burning stump that remains. The Master laughs maniacally and goes to look for the family and subsequently sacrifices his first wife.
The family runs off into the desert. When a rattlesnake appears in front of them, Michael shoots it, attracting the attention of nearby deputies. Margaret and Michael are later convinced to return to the Master's house, where the Master welcomes them. Michael fires several shots into The Master's face at point-blank range, but they have no effect. The screen fades to black, likely indicating that The Master has again applied his hypnotic power.
Later, an entranced Michael greets two new lost travelers. Margaret and Debbie have become wives of The Master. The film concludes with Michael echoing Torgo's line of "I take care of the place while the Master is away." The production credits are superimposed over past scenes from the film with the words: "The End?"

Question: What does Margaret hear that frightens her?
Answer: Howl