It could be the belt tensioner - you can watch it, when it is making the noise it will vibrate badly.
You don't have the remove the belt from the compressor to turn it. Just grab the front of the clutch and give it a turn, this turns the inside of the compressor. This probably wont tell you anything unless it is a bearing then it might feel kind of rough. I have found that making a diagnosis from the way the compressor turns is a bad idea.
You could carefully put a long screwdriver (2' or3') on it while it is running to verify where the noise is coming from, or buy a stethescope at the local parts store to help locate the noise. But definitly check the tensioner.