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There are two identical, small vents...one on the drivers side and one on the passenger side...in the dashboard. While driving with a passenger, have him/her hold their finger on the vent....
If this vent is not causing the disturbance I'm afraid I'm out of ideas on this one. If it is this vent...it can be fixed but will take some doing as I've been fighting it for a while and only have it 75% gone. Worse when the temps are cooler and first thing I hear when I start the car...at idle. If it is coming from a door panel..maybe take it to an audio shop and get it dynamatted.