My truck ('99 Ram 4x4 QC 5.9) has this hissing sound. It does this when changing gears for like 2 seconds or so, but i do not think it does it all the time. Truck runs great, other than a misfire that the dealer is going to fix (just bought truck about 2 months ago). Do any of your trucks do this? I asked someone who used to be a mechanic and he said that his chevy does that when he has the air conditioner on ( i have an air conditioner leak, by the way, don't know if that will cause the hiss). He said that it probably has something to do the emission mechanism on it, kind of forgot exactly what he said. Does anyone know what causes this hissing sound?

Thanks