1973 film The Exorcist what does Chris McNeil think she is hearing in the attic?

In the 1973 film "The Exorcist," Chris McNeil, the young girl possessed by the demon Pazuzu, doesn't actually hear anything in the attic. It's her mother, Karen, who hears the sound of a heavy breathing coming from the attic. This sound, later confirmed by a priest to be the breathing of "something very evil," becomes one of the first clues that something is terribly wrong with Chris.

While the film focuses on Chris's terrifying behavior and the exorcism, the attic scene is crucial for establishing that something sinister is present in the house and that Chris is not just suffering from a normal illness.