see the little button on your climate control that looks like an arrow going around in a circle? that will close the vent that allows in outside air. try pressing that and see if it solves your problem. I don't know any other way to close the vent....maybe do that and turn your climate control off right after and it will leave the vent closed??
also, mine isn't "loud" so check under your car and make sure all of the plastic skids that are supposed to be under there are actually under there. if you're missing one or two of those that will make the car louder.

 

its air coming in from the plastic where your mirrors attach i believe there is a writeup to fix it i just dont remember where...

 

I had major problems with this kinda thing. The is a single bolt that runs from inside the car behind the tweeter to outside. There is a rubber gasket that is supposed to sit tight on the bolt. In my case at the right speed air would blow around the bolt and blow on the back of the tweeter. This made it sound a lot louder than it should be. I did 2 thinks. I filled around the bolt with gasket sealer and I made a worm of detailing clay and pressed it in the back edge of black triangle that mirror mounts to on the outside of the car. It made a huge difference.