She later said she wasn't sure of the date under cross-examination by lawyers for the American student and her Italian former boyfriend who are accused of stabbing 21-year-old Meredith Kercher in 2007.
Nara Capezzali, whose apartment in Perugia overlooks the one where Kercher was slain, also testified that shortly after the scream she heard the steps of at least two people running.
Defendants Amanda Knox, of Seattle, and former boyfriend Raffaele Sollecito deny wrongdoing.
Capezzali made a screeching noise to imitate the scream she said she heard from her living room's window.
"It was a prolonged woman's scream," she said. "It was not normal, it made my skin crawl. Every time I walk near that window, I hear it again."
Capezzali testified that she heard the sound of running on a metal staircase near a parking lot as well as heard footsteps running through leaves and gravel. She said she looked out of the window but "did not see anything."
Kercher's body was found Nov. 2, 2007 in the bedroom of the flat she shared with Knox.
When defense lawyers pressed Capezzali, she began sounding confused and said she couldn't be sure of the date she heard the scream.
Knox and Sollecito were escorted into court Friday looking relaxed and smiling as they greeted their lawyers.
Prosecutors allege that Kercher was killed during what began as a sex game, with Sollecito holding her by the shoulders from behind while Knox touched her with the point of a knife. They say a third man, Ivory Coast national Rudy Hermann Guede, tried to sexually assault Kercher and then Knox fatally stabbed her in the throat.
Guede was convicted of murder in a separate trial last year and sentenced to 30 years in prison.
More Perugia residents were expected to testify Friday about what they heard or saw the night of the murder.