Aw, I must be the only one who likes it when people scream in the stretching room! As long as they are not overly loud and theatrical about it, but really play it genuine. It DOES give me a creepy feeling, to me it adds to the atmosphere. And I have dragged scared little kids through that room, one of whom came out crying, so I know how that's not very fun, but it's a haunted house for goodness sake! If I didn't think they were tough enough to take a few screams or surprises, then I wouldn't take them on the ride.
Sometimes I let out a short little scream in the graveyard portion of the ride when the ghost jumps up from behind the gravestone. I know it's there, but I love the payoff of the delicious anticipation-- it's fun for me to play it up like I'm really scared, it gets me more into the spirit of the mansion. Like the episode of South Park when Cartman is riding through the mansion haunted ride and screaming and laughing at all the ghosts jumping out.
ETA: I have never had the experiences some of you have had, such as people screaming through that entire portion of the show, talking over the ghost host, screaming really loudly or obnoxiously on other rides. I think it's pretty obvious when someone is deliberately trying to be obnoxious, or just getting into the spirit of things. I've seen tons of the latter, and have never had even one experience of the former that I can recall. But I see that perhaps DL is a little worse in this regard than WDW? This may be because MOST of the local Orlando teens (and teens in other parts of the state) would rather skip over the Magic Kingdom in favor of riding the Incredible Hulk. But even with that theory, I can't recall any truly obnoxious behavior. Maybe I just have a higher tolerance level for these kinds of things! The only thing I have ever seen that I felt was truly obnoxious was parents screaming, not at rides, but at their crying and overstimulated children. Talk about grinding my teeth. Urrrghhh! That just always sets me on edge to witness. Yeah, I get it, you've lost your temper-- but your children are not the only ones who are forced to be an audience to your histrionics!