My truck was sounding like that last winter. Turned out to be the vacuum lines and the IWE teeth bent. I honestly drive the worst highway in North America so it bears my truck to crap. Vacuum lines cracked because we had a cold stretch of -50 to -65c for a week and a half.

DG

 

Since my noise goes away entirely when I engage 4x4,im fairly certain it's either vacuum related or a failed iwe. I unplugged the vacuum solenoid with the truck in 2wd and had vacuum. I clicked over to auto 4x4 and vacuum stopped just like it's supposed to so that part is good.

I picked up a vacuum guage tonight and will pull a vacuum on the passenger side iwe this weekendaand see if it leaks down. It looks like replacing the vacuum line would be a real bear since it's a pretty lengthy run from the passenger side tire, part of me hopes it's the iwe despite the much higher cost just for the simplicity of replacing it.