ac compressor noise

I know what a whining automatic transmission sounds like. And that's the sound I heard. It also has the sound like you hear coming from the power steering when it has no fluid. But the sound was coming from the ac compressor. Not a screeching sound. And it is quite loud. But, the strange thing; it makes the same sound even when the compressor is not engaged. Friends, all have an opinion. Everything from the clutch bearing, to it being low on freon; and/or compressor oil. How do you add an oil to a car compressor? I thought that the freon was an oil. But; hey; what do I know? That's why I'm here. I'm on disability and trying to save a few bucks and do it myself. Depending on if it costs to get a reply; then you may have to wait; to reply; until the 3rd of the month when I get my disability check.