The detail from the story that best supports the conclusion that the noise Amy heard was really her alarm clock is: "The whistle and the alarm clock made the same sound."
This statement directly indicates that the sound Amy heard was consistent with the noise produced by her alarm clock. The fact that she was able to turn off the noise by reaching toward her nightstand further confirms that the source of the noise was indeed her alarm clock.
This sentence directly compares the sound of the whistle to the sound of the alarm clock, suggesting that they were identical. This makes it more likely that Amy was hearing her alarm clock, rather than something else.