Air Conditioner Chirping.

I have a 2004 Nissan Sentra 1.8L, When I turn the Air Conditioner on, it makes a Chirping noise, It still blows cold, works good but the Chirping sound is annoying. When I turn it off, It stops. What could it be? Also, When I start it up, sometimes it takes a time or two for it to actually start. It runs good, has 45,000 miles on it. Just has that starting thing going on. Once or twice when it has turned over, it kind of starts but spits and sputters and I shut it back off, then try it again and it starts right up. Any Idea what that may be? Thanks!