i know you said you turned off the AC but does it also happen if the fan is not running? i have had the same type of problem, and this is reported if you search for it, mine was with leaves and trash fallling down through the cowl. The drainage of water from the botom of the windshield actually pass by the vent air intake and there are places where more than water can find its way to your vent intake. if you pull the cabin air filter out, located on the right side under the glove box. you will need to remove a piece of plastic, there is a post that details how to do this w/photos. Be prepared for leaves and other sorts of trash that have collected to be there when you remove the cabin air filter. Be sure to pull your mat out and put a drop cloth or large towel to collect the trash. After removing the filer clean off the debri and reinsert the old filter hold it in place or replace the plastic strip that holds it in and turn the ignition switch to the on positon and turn the fan on high; this will blow out even more leaves and other collected items. Replace with a new cabin air filter and if you still hear the noise you will want to drop the fan blower motor next, in the same area, again there is a post on how to do this as well. There maybe somethting hung up in the fan itself. Good luck!