This only happens when in gear.

As you accelerate and the rpm's increase you hear a little like Shhhhhhhh, which sounds like it is coming from the left airconditioning vent, but it is not the fan cause you even hear it when the fan is turned off.

And only when in gear, if yyou pop it into neutral while coasting on the highway it goes away, as soon as you get in gear you hear it again.

The sound increases with the rpm. But again, ONLY when IN GEAR...

Weird......