I've been browsing the voice acting section of Newgrounds (and in extension some animations), and what can I say, I've heard some people fake a scream.
You should know, that it's noticable (if you're hoping it's not) and really jarring once you spot this. Please don't do this. Either scream properly or don't take a role that neads screaming or yelling.

That being said, my audio interface will be delivered to me tomorrow, maybe I'll show you how it's done :D