First, I'm sorry that I didn't understand the first time.
What you are dealing with is a vacuum leak. The vent doors that direct air flow are actuated by engine vacuum. When you reach that speed, the vacuum is lower in the engine. If you have a leak, the air flow direction will change.
Check under the dash for a dry rotted, damaged, or disconnected hose. Also, you may want to check under the hood for the same. If you are lucky, with the engine running, you may hear a hissing sound where the leak is located.
Let me know what you find or if you have other questions.
Joe
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Friday, October 8th, 2010 AT 12:29 PM