Why does my Spectra's AC make so much noise, and then the noise disappears when the clutch is pressed in?

When I purchased my Kia it was in the winter in the Northwest, I didn't even thing of checking the AC. When it got hot out, and I used the AC for the first time, I noticed that it made a considerable amount of noise (rhythmic, not screeching, almost like a belt is hitting something?). The noise goes almost completely away when I press in the clutch. When I turn the AC off the clutch also feels looser. I don't understand why the clutch and AC affect each other so much. What could be causing the noise, and why does clutch make such a difference?