Why do belts slip?
A car is equipped with an A/C compressor driven by a V-type belt. As the compressor cycles, the belt squeals briefly. A check with a belt tension gauge indicates the belt is correctly adjusted.
A V belt that is not wide enough will run on the bottom of the pulley providing very little belt friction regardless of the correct tension. V belts get their friction by wedging themselves into the V groove.
What belt squeals when I start my car up?
An overcharged AC system condition can cause the high-side pressure to be so high as to resist the start-up of the compressor.
In the long run, the clutch or belt will usually fail.