i have same problem too, i don't think its my vent, the sounds come from the side view mirrors.
Check the TSB's under Tech Info. --> wind noises (Mirror
BTW The back airvents (under the rear lights) or only used when you close your hatch or door (to let the pressure out). You can't disable it by pushing a button. This airvent vents out under the rear bumper. It normally has a plastic flap covering it. This flap can only go outside and so decrease pressure in the car when closing the hatch or so. This flap reduces the sound from outside the car. Just check if it is still there (mine wasn't at one side).
Did you remove the floormat in the trunk? This is also some sound isolation.
Did you remove any panels in the rear?
Chenk all that, if it still too loud for you, turn up your radio. Mine is pretty quiet now after some modifications.