I'm guessing you can pull on the handle to shut the vent window tighter and noise goes away? Most of my friends (and me) would fold a piece of thin cardboard and force it under the handle to stop it. Being young we didn't know (or care) about fixing it as long as the car ran right.
As far as a better cheap fix (if this is the problem), maybe gluing a small nylon washer on the tip of the handle?



Tim