Emma Molloy – now 36 and a housing warden – told the High Court in Edinburgh she was afraid to look out of her window after hearing the noise.
She told how, at the time more than 27 years ago, she lived just round the corner from Elaine’s Greenock home.
Her mother worked night shifts at National Semiconductors and her cousin Avril Clark, 44, was babysitting for her.
Ms Molloy said she described what happened on 3 June, 1986 to police investigating Elaine’s murder.
The trial heard how she told them she had been trying to listen to her mother’s radio alarm just after midnight but switched it off because it was only buzzing.
“I then heard a loud scream coming from outside. It sounded like a woman,” the mother-of-two said in a statement.
“I then heard a loud bang outside. I was frightened and hid under the blankets for about five minutes.”
Questioned by defence QC Donald Findlay, Ms Molloy agreed that ten years later she again described the night to police, but only told them about hearing the bang. The scream seemed to have been forgotten.
Administration assistant Ms Clark said on that Sunday she and her cousin had watched a film, California Suite, on television. With the help of TV listings from 1986, the trial heard that the film had finished just before 12.40am on 2 June – the date schoolgirl Elaine, 16, was found dead.
Minutes later, after she had gone to bed, Ms Molloy came into the room and told what she had heard. “I dismissed it. I just thought it was a noise she had heard on the radio,” she said.
And the court heard how health care worker Maureen Gray helped a police artist to draw an auburn-haired man she saw in a common stair just hours before the teenager’s naked body was found near her home.
She later told murder hunt detectives the pale man who was breathing heavily and smelled of drink made her feel “apprehensive.”
Mrs Gray, now 53, said she and her boyfriend Joe – now her husband – were watching late-night coverage of the Mexico World Cup on TV in a flat near Ardgowan Street. She said she later went down the communal stairs and saw a male standing there.
She said: “He looked quite agitated. When I passed he was breathing heavily. He was sweating and I thought he had been sick.”
Mrs Gray described the male as “a mess” with his shirt hanging out and his hair wet.
John Docherty, 49, of Dunoon, denies the schoolgirl’s murder. He claims at the time he is alleged to have stripped and strangled the 16-year-old, he was at home with his parents, who are no longer alive.
Docherty has also lodged a special defence of incrimination, claiming the culprit might be among a list of 41 names taken from files of the police investigation into the murder.
The trial continues.