I was wondering about that too, with the example of Pony Express in particular. It's not a very tall or fast coaster, but its final turn still had to be enclosed. If that is the sort of requirements they are operating under, how would they be allowed to build a Giga?
I don't think the track rumble is the main issue. It's the screams from riders. Human screaming is a sound the ear is evolutionarily trained to pick up and be alert to, so it is very jarring. It is a somewhat easily muffled sound because it's high-pitched, but it also travels easily. The first drop of a Giga would cause very loud screaming, and if the final turn of a motocoaster is unacceptable, what would it take to fit a Giga in the rules? I mean, enclosing the drop would take away most of the point, wouldn't it?