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.

A: Hello there, thank you for asking about you...

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・Save up to 30%

Get a quote

What others are asking

Q: it spitts and sputters when setting and does the same thing when shifting gears and drinks the gas

The engine needs the proper fuel and spark to operated correctly. You may want to see if you notice any lights illuminated on the dashboard. The possibilities of this condition are an engine misfire, which could be caused by a...

Q: Vehicle makes rattling noise when idling

Based upon the information provided, an inspection of the drive belt and air conditioning system should be performed. The squealing noise indicates your drive belt is under strain. If the belt breaks, you will not be able to drive your...

Q: Car chokes after starting

There are two options for the engine for your car. They are both 3.0 v6’s that are vastly different. If you are talking about oil in the spark plug tubes on the DOHC v6, that indicates a bad valve cover...

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...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...