Skip the auto shop - Our mechanics make house calls

Q: Knocking sound in passenger side under dash

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car engine revs way up immediately upon starting engine.

Common causes of high RPM excursion at start-up are typically: large vacuum leak, throttle plate stuck open, cruise control stepper motor stuck in one position; faulty throttle position sensor, and/or faulty idle air control motor or solenoid. If you request...

Q: Air conditioning blowing hot and won't keep charge

If the AC system won't hold a charge, there is a leak somewhere in the lines or other components. Leaks can be tested for by using a UV dye. Due to the danger involved, I would recommend having a mechanic...

Q: Car won't start

The charging system is not recharging the battery. This is why warning lights are on the dash and the vehicle loses power. It will jump start and run until the battery runs out of juice. The most common problem...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...