Weird humming noise (possible 4x4 problem)

I have a 1995 Blazer LS 2Door with 4WD. The last week i was driving around 50mph when i noticed a strange humming noise coming from the front of my truck. It almost sounded like someone was holding a piece of cardboard against the treads of my tire(if anyone can imagine that sound). The sound got louder and lower pitched as i slowed down. Then it went away at under 25 mph. I got out and check my tires and nothing was touching them. This has only happened one other time in the past 2 weeks. My dad said it could be the 4WD trying to engage but I wanted to see if any of you guys have some suggestions. Thanks.