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On an ordinary day, Claire Kutty, a secondary school English teacher in her early forties, hears a low, rumbling sound while driving her daughter Ashley to their school. This innocuous noise starts to cause Claire migraines, nosebleeds, insomnia and even visions. No obvious source or medical cause can be found – and no-one else can hear it.

After weeks of feeling increasingly isolated and on edge, Claire is approached by one of her students and Ashley's classmate, Kyle Francis, who tells Claire he can also hear the sound.