That is a hard one since loud and scary ar a bit subjective. I didn't understand why my son hated fireworks - we were very far away and couldn't 'feel' any booms so I just didn't understand. My friends dd (ADHD and some other issues went with us. She rode Dinosaur and definitely didn't enjoy but didn't freak. She was practically crawling on top of me in the great movie ride. She was so excited she kept standing up on splash mountain, after refusing to go on several other rides. I sat out of honey I shrunk the audience, the haunted mansion, and several others. She expects dark to be loud and startling. She gets through what she likes by holding her fingers over her ears. And panics otherwise. Would you believe the Snow White ride is the scariest thing at Disney World for my children?
So, my point is. While we can give advice as to our individual children's reactions, we or the author of a book can't actually give you an accross the board scare rating. While volume might be easier to rate, it is still subjective as to what is a bothersome noise. Some kids it's all noises, other kids it is certain types of noises.
Your best bet is to check the disabilities boards, amazon.com and the like for disney with disabilities books (browse in the bookstores). Several of the regular tour books do give this kind of rating - but from the view of a person with no issues. My regular book is the Passporter Walt Disney World Resort. Mine is several years old. However, I did use it for hints on things to avoid especially with the friends child. And I haven't looked at a recent one to see if this has changed. The book rates many of the things from several points of view. Rides are rated from each the husband, wife, and child's point of view. There are also hints for the rides about what ages the ride is geared for. I do see some that note the ride is loud, dark, scary, etc. I have purchased 3 different books, and this is the only one I still own.