Your car’s AC system is designed to keep you cool and comfortable when the temperature rises. It’s also designed to operate quietly and unobtrusively. There should be little to no noise from an air conditioning system that’s in good working order. However, if you’re hearing a rattle when you turn the air on, it could be any number of different problems.
While your air conditioner is technically a separate system, it’s tied to the rest of the engine by the serpentine belt. The serpentine belt is what’s responsible for turning the pulley at the AC compressor, and pressurizing the lines. The compressor engages/disengages by an electromagnetic clutch.
If you turn on the AC and immediately hear a rattling noise, there are a few potential causes:
Compressor: If your compressor is beginning to fail, it can make a rattling noise.
Pulley: If the compressor pulley bearings are failing, they can make noise (generally a grinding, roaring or squealing noise).
Belt: If the serpentine belt is worn, it can slip when the compressor engages, causing noises.
Idler Pulley: It’s possible that the noise is actually from the idler pulley if its bearings are failing (the noise would start when the compressor engages due to the increased load on the engine).
Compressor Clutch: The compressor clutch is a high-wear item, and if it’s worn out, it can create a rattle during operation (some vehicles can have just the clutch replaced, but some require replacement of both the clutch and compressor).
There are many other potential sources of the noise as well. When the AC turns on, it puts the entire engine under a heavier load. This increased load can cause rattles from things like the power steering pump pulley, loose parts (even a loose hood prop rod can rattle under the additional vibrations created by operating your air conditioner).