Air Pressure Release Noise

OK, I will try and make this as simple as possible to explain my problem. I have a 2001 GMC Pickup Truck..V8 engine. 3/4 ton.. under warrinty for about 4 more months. Took truck in for noise coming from passenger side. Sounds like air releasing, something like big truck releasing air brakes. When weather is warmer.....like over 40 then it does it. Will not do it when I am standing still....only when driving. Also does not do it if I shut off controls to heater/air. Took to dealer and they were not able to duplicate sound. Of course it was colder out at that time. Today, temp was over 40, so I took write up guy for test drive, and he finally heard noise. When its warmer out, noise happens about every 40 seconds or so. And the only way to describe sound is just a loud release of air pressure. Hope you can figure this out. Going to drop off again to service. thanks..

Just a update for anyone experiencing the same noise......Problem fixed. Cause was Air Conditioner pump replacement. Apparently when it cycled, built up too much pressure, causing it to blow out, making air pressure sound.