Never been sure on this myself.
Its always at a regular timing, it doesn't sometimes happen and sometimes not. I always assumed that the second she goes to scream she will start dealing damage immediately, however her scream at that point isn't yet audible.
You could hear it alot better on the Christmas Sirens scream, since she had a dying tone.She would start to scream, then suddenly cut it off.
Not sure if it detracts from the game at all, it does tend to do a very miniscule amount of damage, indicating she is kinda building up to her scream, and hasn't quite fired off the full effect yet, but is still causing vibrations in the air.
Since its an effect that happens everytime I'd guess its intentional, but its hard to say
