Re: An hissing sound coming from the vents !
Sounds like leaf debris in the squirrel cage blower to me. Raise hood, remove all the torx screws that hold the plastic cover under the wipers, and pull it free.
Then, vacuum the blower and passages out under there. My Graphite got to making noises and when the guy put the new windshield in, he saw the crap and cleaned it out for me.
No more noise.