C320 AC Siren Noise
 

So I have a 2001 C320, I've been recently having this issue with my car. Every time i turn on the air conditioner and have my car stopped for a long time an alarm goes off sort of like a siren and will not stop until i turn the car off. Ive noticed this only happens when my car is stopped. for example if I'm at a red light or sitting in traffic for a long time the siren starts. does anyone know what this could be ?
thank you