I have the 2005 Crew Cab Sierra with a 5.3. When I'm sitting at a light idling, the AC makes what I consider an unusual amount of noise. It's kind of a mix between a gurgle and a slight metallic noise. I don't remember it being this noisy when new and I've never had another vehicle that had a noise this noticeable. I only notice it at idle, but of course when driving the vehicle itself makes enough noise to block it out. Any ideas?