Q: 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.