Here's why:
* The A/C compressor clutch engages and disengages: The compressor is the heart of your A/C system. It's responsible for circulating the refrigerant. A clutch mechanism engages the compressor when you turn the A/C on, and disengages it when you turn it off.
* The noise is likely bearing related: When the A/C is off, the compressor is disengaged, but the pulley is still spinning. If the compressor's bearings are worn or damaged, they'll make noise as the pulley rotates freely. The noise typically goes away when the A/C is on because the clutch engages the compressor, and the load on the bearings (due to the rotation) minimizes or changes the character of the bearing noise. Think of it like how a squeaky wheel can sometimes quiet down if you put weight on it.
* Other possibilities (less likely): While less probable given your description, other possibilities include issues with the serpentine belt tensioner or pulley, but these usually make noise regardless of A/C status.
What to do:
It's crucial to address this noise. Ignoring it could lead to complete compressor failure, which is a costly repair. You should take your Sunfire to a qualified mechanic for diagnosis. They can inspect the A/C compressor, its bearings, the clutch, and the serpentine belt system to pinpoint the exact cause and recommend the necessary repairs or replacement. Driving with a failing compressor could damage other parts of your A/C system.
