MAGLIOZZI

Dear Car Talk: I have a 2021 Buick Envision. Lately, when I turn the air conditioning on, a low, unearthly moaning sound comes from under the dashboard. It’s the kind of sound that, if you heard walking by a cemetery at night, you might run faster than you ever had before.

It lasts 6-7 seconds, then doesn’t make the noise again until the next time I turn on the air.

The AC and the vehicle otherwise run fine. Do I need a mechanic? An exorcist? — Jim

I’d get both, Jim. Just to cover all the bases. Let me be clear from the outset: I don’t know the answer without being able to stick my head under your dashboard and hood and listen for the noise. But I’ll give you three possibilities.

One is that it’s due to the initial movement of the coolant when you turn on the air conditioner. When you fire up the AC, your coolant has to be pressurized. And it can make a groaning-hissing sound when it passes through the expansion valve near the dashboard. This becomes more likely if your refrigerant level is low.

Another possibility is that it’s the sound of the compressor starting up. Particularly in hot weather, the internal valves of the compressor can groan for a couple of seconds as they adjust to the pressure.

And then my final guess is that you have a noisy blend door. Blend doors are flaps inside the ducts of your heating and cooling system.

Every car since around the Roosevelt administration (Franklin, not Teddy) has had a way of directing the flow of air inside the cabin. When you decide you want more hot air on your feet and less on your face, in this car, you’d go to a screen on your car’s climate display and push a button that has arrows pointing toward the floor.

And the way the car obeys your command is by using blend doors. Once activated, the blend doors move inside the ducts and close off (or partially close off) air to one set of ducts, and direct it to your preferred set. These flaps are often controlled by little servo motors, powered by electricity or vacuum, and it takes a few seconds for the door to move to its new position.

So, a door could be sticking, a servo could be weak, or a petrified piece of cheddar cheese popcorn could be blocking a door from closing or opening all the way. But I’m guessing the noise is related to one of the three possibilities I listed, rather than the restless ghost of David Dunbar Buick.

If you have an extended warranty, you can take it to your dealer and ask them for a positive and definitive diagnosis. If you’re not under warranty, I’d just have your mechanic check your refrigerant level to make sure it’s not low.

And then, as long as the system is working well, and the groaning doesn’t get worse, just use the noise to scare neighborhood kids on Halloween and call it a day, Jim.

Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.