It sounds like you are dealing with a vacuum leak. The vents are actuated by vacuum. If there is a leak and the pressure drops too low, the air flow will change directions. Check under the dash and the hood for leaks. If you are lucky, with the engine running, you may hear a hissing sound.
Let me know what you find.
Joe
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Thursday, July 15th, 2010 AT 11:41 PM