There's a few different things that can be happening.
The bearings on the ac compressor can be going. Often you will see metal flake below it when they go.
The clutch can be spaced incorrectly and cause a noise. There is a tool to space the clutch. Most guys won't try this though, they'd rather sell a compressor.
Same belt as the rest of your driven accessories. If the sound changes when you try the AC, then likely it's trying to work. Get out and take a look at the front of the compressor where the pulley front is.. does it turn when the ac is on? It shouldn't turn when the ac is off.
