I tend to disagree. 'The scream' was widely
reported in the media after Amanda's statement was released (perhaps the handful of 'ear witnesses' who later came forward claiming to have heard a scream were influenced by that early press coverage). Guede would likely have read that coverage, so it doesn't seem all that unlikely he decided to weave this apparent 'truth' into his own account.
Guede also needed 'the scream' to have happened to make his story work. He said he was in the bathroom listening to his ipod when he heard it - a scream loud enough to be heard over the sound of the music - and it was this which caused him to leave the bathroom and come face to face with the mysterious attacker, and to see an Amanda-shaped 'shadow' out the window. Without the scream, he has nothing to explain why he left the bathroom just at that moment. So perhaps he simply incorporated the scream into his story because it was convenient, not because it happened.
Having said that, it's certainly very possible there really was a scream, and that Guede mentioned it because he worried someone else had heard it. I just think that to say "if Rudy said there was one there was. That's it" is greatly overestimating the certainty of it.
The discussion was actually about whether the scream happened when Meredith first saw Rudy, or immediately before she was stabbed (perhaps that's what you meant by the start of the violence; I think that moment was probably the end of the violence rather than the beginning, though). But when the knife was produced is certainly one more possibility, even if I wouldn't put a 98% probability figure on any of them.

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