For months, whatever valve or door that directs air to the center vents (the ones with the manual sliders) seemed to have a mind of its own. Last summer, I could not get cool air to come from the center vents at all. I tried switching the defrost on and off, the recirculated air switch on and off, etc. but nothing worked. Then, one particularly hot day, it opened and behaved normally for months.

Recently, however, I'm hearing a "thunk" inside the dash when I start the car, like some vacuum controlled door is slamming shut, and I'm only getting fresh air from these vents. Heat or air conditioning is only going to the windshield or the floor. Also, the blower is making a faint, buzzing noise, like a piece of foam rubber is caught inside the fan.

Vacuum leaks are the most likely cause. You need a vacuum gauge to check for leaks at the A/C switch-over valve.

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