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Q: Knocking sound in passenger side under dash

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When I start my car the only AC options on 3 and 4 (Semi high and high). The other two options 1 and 2 do not work. When I want the AC to turn on I have to hit the dash underneath where the passenger is. Once that happens there is a knocking sound that comes from that same area. What would I need to do to get every AC option working and to stop the knocking?

My car has 145000 miles.
My car has an automatic transmission.

Hello there, thank you for asking about your 2008 Mercury Sable. This sounds like it may be related to one of the AC vent doors. These can often times get hung up and create this kind of sound when they are trying to open and close but cannot. Try turning the vents to other options (i.e. from defrost to floor and back) to see if this changes it or makes the noise go away which can often times occur from getting a door stuck loose. If this does not work, I would recommend having an expert from YourMechanic come to your home to diagnose your car’s noises and make the necessary repairs.

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