[The major reason Dipper doesn't scream is that he's going too fast and he's not sure the sound would keep up and that's a disturbing enough thought that it keeps the noise inside, though there might be a bit of whimpering that escapes. It's just... it's just a lot faster than a person should go, and though Dipper would deny being scared with every fiber of his being, it's a little terrifying. He clings to Tommy's forehead as hard as he can and tries to remember to look around, but...
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Well, honestly all he sees is blur.]