2007 Z4 3.0Si Coupe. I generally have the air flow set to exit the dash vents and turn the fan on/off as needed. I have noticed several times now that if I change the controls to redirect the air to the windshield defroster vents, the air flow does not change. If I turn the fan speed up to max, the air sounds like it's being blocked and the fan is straining. After switching the air flow control between the defroster, dash, and floor vents several times, eventually I can hear the air slowly being diverted to the indicated vent until full flow is acheived. The air flow control then works fine for a couple of weeks. It seems that the mechanism which controls the air flow is sticking. Is there a quick fix, such as an easy to reach part lubrication, or is it off to the dealer?