Geez, that's odd. Not too many components in that spot other than vacuum pods that control the climate control flaps. I guess it is possible that you have a vacuum leak there.

If you track down the vacuum line that feeds that system and plug it, you can see if it then goes away. If so, you have a vacuum leak.

Does pushing the throttle hard make it stop? That might indicate a vacuum leak as well.

The only other thing I can think of is that the seal at the bottom of your windshield is shot and you are getting air blowing past it making noise, but that is really a shot in the dark.

-tp