It's probably the clutch. Get a used whole compressor/clutch from a junk yard. I just bought one for $25 plus $15 for shipping. Comes with a 30 day warranty. I looked it up on http://car-part.com/.

If you don't want to evacuate, replace your drier, and re-charge, you can replace the clutch without removing the lines. Just swap over the clutch and see if that fixes it. It's a tight space, but can be done.