If your vehicle is showing loud squealing noise from under the hood when a/c is turned on, here are the most likely causes and the parts you'll need to fix each one.
The belt that drives the A/C compressor stretches, dries out, and glazes over time. When the A/C turns on, the sudden extra load causes the worn belt to slip on the pulley, creating a loud squeal.
The spring inside the belt tensioner weakens with age, failing to keep adequate pressure on the belt. This lack of tension allows the belt to slip when the A/C compressor engages.
If the belt was recently replaced, it may not have been tensioned correctly (on manually tensioned systems) or a slightly oversized belt was installed, leading to immediate slipping under load.
The magnetic clutch that engages the A/C compressor can wear out or lose its proper air gap calibration. When this happens, the metal clutch plates slip against each other instead of locking, causing a metallic squeal.
The internal bearings or pistons of the A/C compressor are failing and locking up. This creates massive resistance, forcing the belt to drag violently across the stopped pulley.