Your post made me think about how that scream must have sounded to the neighbor's son, that is, to cause him and his mom to be alarmed to that degree.
I study acting/voice acting and there are many ways to scream and defining characteristics. Wish we could ask questions of the neighbor's son.
We've talked before about that oftentimes we hear children screaming when they play, or the type of screams an adult who is scared of mice or spiders, or being playfully thrown into the water would make, or while on a roller coaster.
So, for me, at least, that particular scream would have had to elicit in him that the person was in danger or pain for example.
I would guess it was loud and piercing, but that is merely guessing. So many questions I would ask.
Just some examples:
Was it one long or short scream or a series; for example, was the scream muffled for a moment, then resumed. About how far from the two houses does he think it was. Were there any words contained in it, such as No! Stop! Help! Rape! And most importantly to me is: Did he recognize that it was Holly's voice? If so, when did he realize it was her? And, when did it happen?
As usual, questions.
P.S.: I distinctly remember the scream I made when I was attacked and raped, that is, after I pushed this person away from me.
Also, I remember performing a role where people said they literally were levitated off their seats by my bloodcurdling scream. Now that I think of it, there were several roles where I was required to scream in various scenarios.