I think a big contributing factor is the restraint design. Take a look at Kingda Ka vs. Dragster. Dragster is universally agreed upon as a much smoother ride, and I would argue that is due to the restraints. All of the restraint hardware on TTD is underneath the train and sits very close to the track, while Kingda Ka has all of the restraint hardware (pivot joints, hydraulics, etc.) is located in the column right behind the seat. This added weight, being so elevated above the track, (I believe at least) causes a fair bit of rattling when a train gets up to speed. The same can be applied to newer Intamin coasters like VelociCoaster as well.
I've also noticed on I305 (just because I have the most experience with that) that empty trains rattle far more than a fully loaded/weighted train. Haven't done either of the newer US Intamins yet, but I'd expect that to be the same there.
I've also noticed on I305 (just because I have the most experience with that) that empty trains rattle far more than a fully loaded/weighted train. Haven't done either of the newer US Intamins yet, but I'd expect that to be the same there.