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・Fair and transparent pricing

Get a quote

What others are asking

Q: Air conditioning not working in the front of my SUV

My guess is that the refrigerant level is low (from a leak somewhere) or there is a problem with the AC pressure cut out switch. The control module will cut power to the AC compressor if it determines the system...

Q: AC fan only works on high

This is a common problem when the A/C blower motor resistor goes out. This is an inexpensive part to replace and is found under the dash near the A/C blower motor. This resistor controls the speed of the blower fan....

Q: A leak from power steering pressure line hose

Hello. If that other hose is leaking then it will need to be replaced also. If the section of hose that is leaking is the metal line then that can usually only be purchased at the dealer. This line is...

Related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
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...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.