a/c compresser making loud noise

My a/c compresser is making a constant loud noise, sort of like "errrerrerr" it only does this when the a/c is not on, when the compresser engages it stops. What does this mean, and if its going bad where can I buy one-all the parts store on the net I've seen say the part is not available. Thanks in advance.

Have you checked www.bavauto.com? They sell pretty much any BMW part you may need.