Whinning noise

Hi
l have a 1999 dodge durango 5.9l 4x4 with 178k.
when l backup the truck l can hear a whinning noise. L got someone to put it in reverse while l listened to where it was coming from, it sounds like the transfer case. A couple of day later l started to hear it while driving down the road. One day its there the next day its gone. Any suggestions what is going on with my truck.