'05 Frontier "Instrument Panel" noise.
 

I have a '05 Frontier crewcab and hear a buzzing and/or soft knocking sound behind the instrument panel about middle to driver's side middle of the dashboard. This happens intermittently, so it is tough to get into the dealer and have them locate the problem. Almost seems like there are solinoids opening and closing the dampeners for the ventilation system without my changing any of the contols. Every once in a while the air will just change on it's own from defrost to passanger vent and back.

Any ideas where to look first? or what the problem could be?