My son has a 2007 Impala that has started making a noise I've never heard before. Any time you drive with the AC on, and then turn the car off when you get where you're going (as you do), there's what sounds like a fifty-pound woodpecker trying to drill its way out of the passenger-side dash. You can sometimes silence Atomic Woody for about ten seconds by randomly turning the AC selector to a different setting (this is while the car is turned off, remember) but then he comes back. This goes on for a minute or a minute and a half (although I have to say it seems longer if you're forced to listen to it), then subsides.
While the car is running, AC performance seems unaffected.
Anybody care to enlighten me about what the hell is going on here?
TIA
Servo motor for one of the air doors acting wonky?
You said atomic woody. 
No help other than that. Sorry.
One of the blend/selector door servos is unhappy (I'll hazard a guess there's a limit switch inside that fails), my dad had a GMT400 truck that did this for years before it failed entirely and just kinda blew out every vent after that (temp and fan controls still worked). I've never been in a W body dash but it's probably not worth going after if you can't drop the offender out with just the kick panel or glove box removed.
Probably have a broken gear inside the actuator, had the same sound in the Bonneville. It will get worse, so best to replace it. Should be able to find a how-to online.
Blend air door motors tend to go bad on these cars, the pass side is easy to get at but the drivers side will require some blood.
Start the car.
Switch from fresh air to recirc(or back).. It will make the noise.
Its stripped gears inside the recirc door actuator.
Pull the glove box and you can put your hand on it(to the far right IIRC) to verify.
They switch to fresh air any time it's turned off by default, this is why you hear it when shutting off.
Very common, easy to replace. Not a bad idea to pull the old one, make sure that the door itself moves freely by hand. I have seen animals build a nest big enough on top of the cabin air filter that it keeps the door from closing. You can get it all out by removing the blower motor if this is the case 
Knurled
MegaDork
7/27/16 12:37 p.m.
It is not necessarily stripped gears, if the position sensor fails, the motor will keep cranking the thing trying to put it in its home position.
I want so bad to link to an S.O.D. song about noise here, but it has naughty words and some folks have no sense of humor..
In reply to novaderrik:
Hey Alex, what the...............
The hive totally rules, as always. Thanks to all!
