Just because a bunch of people on the most biased form of commenting system in the world (a car forum) have rattles in their interior does not make it in any way shape or form normal. Rattles are not normal on ANY new car, let alone a relatively well built Mustang.
I personally don't understand how a rattle can get louder when you install an intake that should drown out the rattle in the first place :?::?:
I'm not trying to say it SHOULD be normal but the fact is a lot of people have some rattles which, by definition, makes it normal lol. Unfortunately it's just not economical for Ford to ensure every single piece fits perfectly on every vehicle they make so slight rattles are normal. If the car is constantly rattling or the rattling is excessively loud then I would agree that is not normal.
As far as the intake thing goes, sound is created by pressure waves which have the ability to rattle pieces of a car. Especially when they are a low frequency and at a high volume (think of a sub-woofer rattling a trunk). Now the intake doesn't have anywhere near the same amount of power but the concept is the same. When compared to the stock air box, the new intake is now creating a lower frequency at a higher volume. Then you factor in that it is only sitting a couple feet away from whatever is rattling in my dash and that makes the problem worse. I only ever notice it when my engine is under a slight load (2200-2900 rpm) which is where it sounds the deepest.