AC vents making noise when AC is turned off

When I turned off my defroster this morning, the vent(s) started making a loud clunking noise, kind of like knocking, while I was driving. When I turned the motor off and started up again, the noise continued. What can I do to stop this annoying racket?

A bad mode door actuator can cause that.



You may have a bad blend door actuator or a mode door actuator. They open and close flaps to direct air flow to different areas and control the temperature. They contain plastic gears. If a flap gets stuck or hard to move a gear may strip and cause this. If the climate control module detects a problem it automatically tries to put it to defrost. Try listening under the dash just above where your feet are. If that’s the source of the noise then you probably need to have that actuator replaced and check that the flap isn’t obstructed.

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GM is pretty notorious for blend door and mode door actuator problems. My 2005 lesabre sounds like a tap dancer is behind the dash sometimes. 2005 Sierra has an occasional tap that I believe I’ve narrowed down to the mode door actuator. 180k miles or more on each of them and they’re 16 years old, so I’m not too mad at GM.

Change the temp and listen for changes in the sound. Change the mode and listen for changes in the sound. Might help narrow it down before you get under the dash.

You’re supposed to come to this forum to declare that the mfr should do a recall of your 16 year old vehicle for a non safety-related problem, and that you will never again buy a GM product because of premature component failure.

Get with the program, man!
Don’t you know that manufacturers are responsible for every problem, no matter how old the vehicle is?
(If not, why do we get so many posts assuming that totally bizarre line of reasoning?)


AC is still cold. Turn the radio up to give the tap dancer something to dance to. Works for me! Lol

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It seems to be particularly acute with Asian cars, notably Toyota. Some people seem to expect those cars to run hundreds of thousands of miles without repairs.

Good comments above, likely blend door motor malfunction. If not that, could be the normal, and nothing to do w/the hvac doors. For example an ABS self-test could make a similar sort of sound.