Did you ever find out if it was your leveling kit? I have one on mine and it makes the hissing noise

 

I went through two sets of actuators on my '09 within a year of getting it. Truck had 114k when I bought it, and when flipping into 4wd you'd hear hellatious chonking noises from one or both of teh actuators half-way grabbing teeth. In addition to the 4 actuators, also replaced the solenoid, all check valves and most of the plastic line. In the end I ripped all of that garbage out and replaced the actuators with solid ones from Dorman.
From what I had found during the probably dozen times I had one actuator out, it appears that when locked in, there's less than 1/8th" of actual teeth meshing.. honestly it's more like 1/16th". Like, whomever designed this was an absolute clown just to save maybe 1 mpg.

IDK my situation is different from most I guess.. I work from home and don't tow and don't take long trips in it. The super minimal loss in efficiency and increased CV wear, which is a joke IMO, was entirely offset by the fact that I have reliable 4wd when I need it regardless of engine vacuum.

Anyway there's no way it's caused by levelling your truck, it's a ****-poor design at best and in my experience prone to failure.