It does it when you take fatal fall damage if I recall correctly. I absolutely abhor the Wilhelm scream and a few other overused sound effects. It completely destroys any suspension of disbelief I had. Audio engineers often put it in as a sort of running gag, but it's run far too long and has completely worn out its welcome with me. That said, some manipulated variations of the scream are fine and actually makes it seem like the audio guys were being creative. As it stands now, it feels like a fart gag inserted into serious situations.