It is no accident that their stadium is loud; it was built with just that in mind.
The architects hired to build the stadium were specifically instructed to make it loud - despite the constraint of it also being an open-air stadium.
A couple of things that they did to accomplish this was to include aluminum bleachers rather than plastic or padded seats, and a curved roof that causes sound to be retained and reverberate.
In addition the space in which they were allotted to build was much smaller than any other newer NFL stadium, so other modern conveniences such as wider concourses - which as a by-product dissipate noise - are not as prevalent here.