Dear Car Talk:

I think our minivan might be haunted! My husband and I own a 2002 Ford Windstar. It has 97,000 miles on it. About two weeks ago, a strange phenomenon occurred: It started knocking! It started quietly, and then got louder and louder. I was quite alarmed when this began happening. I was pulling out of our driveway, with my daughter in tow. I yelled to my husband in the backyard to come quickly, explaining that something in the van was knocking. My husband strolled over and -- guess what? The knocking had stopped! He said, "I don't hear anything!"

Fortunately, my daughter was able to verify the noise, or he probably would have had me committed! In subsequent days, the knocking has continued to occur at random times. I haven't been able to establish any kind of pattern with it -- it just abruptly starts and stops. The noise seems to be emanating from behind the glove box. Sometimes it is very loud, and other times it is more like a tapping sound. Usually it starts out softly and crescendos. I strongly suspect that we have a poltergeist.

I guess my question for you guys boils down to this: Should we call a mechanic, or an exorcist? Your input will be greatly appreciated!

-- Amy

I'd go for both, Amy. Cover all the bases.

It sounds like you have a stripped gear in your ventilation system. Inside the air ducts that run behind your dashboard, there are flaps -- called blend doors. They change the direction of the air (to the windshield or to your feet, for instance) and regulate how much hot air enters the cabin.

Each blend door is controlled by a small, vacuum-operated motor. That motor (really, an actuator) turns a plastic gear, and the gear opens or closes the little door to which it's attached.

I'm guessing that one of your gears is stripped, so as it tries to move the blend door, it's able to move it part of the way, but then the door snaps back. That's what creates the snapping or knocking sound.

Next time you hear it, try playing around with the ventilation controls. Try moving the knobs that control the amount of heat and the direction of the air. My guess is you'll be able to change the noise, or even get it to stop. Of course, you may find out that in order to avoid the noise, you have to set the heat at full blast directly into your face all summer.

So your other option is to get it fixed. If you're really lucky, your mechanic might be able to reach the offending blend door by removing the glove box. But if he has to remove the entire dashboard, that's going to get expensive. Like, hundreds of dollars.

So when you find out what it costs, you may just decide to live with your poltergeist, Amy. Maybe you and your daughter can use the steady beat of the blend door to practice your rap.