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!