Crackling sound while driving NEED HELP!
 

Hay guys so lately I've been noticing this crackling sound in the morning when I'm pulling out of my neighborhood (almost like liquid evaporating) and I start to take my foot off of the gas at about 20-25 mph. This sound only occurs after the truck has been sitting turned off for about an hour or two, and doesn't occur at speeds > 35 mph. It really only happens when I start to accelerate from a complete stop, but does not happen at a complete stop after driving for about 5-10 minutes. (This might be a completely different problem) I've also noticed while my AC is on and I'm sitting idle, my engine will down rev below its idling point for half a second. Not sure if that problem has anything to do with the noise, but I would also like to get some feed back on that issue as-well. It would be great if I could get some ideas of what could be causing the sound/AC problem and some easy ways to test for it. It would be really great if I could fix these problems especially without having to take it to a mechanic. (03 2500 Silverado 6.0L Vortec)