I have a 07 XTR, and when i have the AC on, i can hear like a hissing noise every so often... it will hiss... go away... hiss... not sure what to make of this. any ideas?

 

mine does same...only really noticeable when AC fan is on a low speed and the radio is off. My hiss comes from the black plastic housing around the evaporator coil in the passenger side floor. My AC is very cold, so I'm not complaining about the noise...haven't had it looked at yet.

 

If your only hearing it with the cab really quiet, it may just be the refrigerant flow through the evaporator coil especially if that's where the noise is coming from.