Q: What Causes a Car's AC System to Rattle When It Is Turned On?

asked by on

What causes a car's AC system to rattle when it is turned on?

The cause of the rattle will depend on whether it is coming from inside the car or under the hood. Typically, a rattle would be associated with the compressor and underhood components.

Let’s take a look by starting under the hood:

When the AC compressor is engaged, typically it uses an electromagnetic clutch which has a specific air gap; over time this can wear or start to slip. This can occur due to a front compressor seal leak which can cause oil from the AC system to contaminate the clutch and allow it to slip over time. The belt for the system may also stretch when under the stress from a weak or defective tensioner which is too loose or tight and may cause noise as well.

Loose, stripped or broken bolts for the brackets that mount the AC are another possible source of rattle or noise along with the idler pulleys for the system. This condition may also be apparent even when the AC system is not selected as the compressor will cycle or run to help assist with the defrost feature in your car. AC systems remove moisture by dehumidifying the air that enters the vehicle during its operation. You may have noticed the accumulation of the water collected during this process, especially on a hot and humid day as the puddle under your car gets bigger and bigger from the moisture removed. This same principle allows the defroster for the front windshield to remove excess moisture to help clear the windshield of fog buildup.

If we look at the inside of the vehicle, we find that a blower motor with a bad bearing or with some sort of debris stuck in it can also cause a rattling or droning sound. Despite its location, the heating and AC system will often get outside leaves or other debris accumulating inside and can even have unwelcomed visitors in the form of mice or other creatures try to make a home inside. This again will be more of a vibration and noise than a rattle. The interior noises will be present whenever the heater or AC are on as the fan is used for both systems. So have the belts, pulleys and system checked for the rattling sound to ensure cool comfort for those hot summer days.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 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!
  1. Home
  2. Questions
  3. What Causes a Car's AC System to Rattle When It Is Turned On?

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 cooling

A number of different things could be causing this issue. The most common cause is that the system is low on refrigerant. This would have been caused by a leak in the system. The first step would be to turn...

Q: Driver side tire makes small scrapping/metal noise

Noises like these can be caused by a few things, including worn wheel bearings. A worn wheel bearing usually gives off a groaning noise that may or may not get louder as vehicle speed increases. In most cases, a scraping...

Q: No blinkers, windshield wipers, AC, or radio

Hello. A few things I would check are the ignition switch and the wiring, as well as the various relays and fuses that control the power to those circuits. Blown fuses or faulty relays can cause the symptoms you are...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
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...
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...