Got some snow yesterday and used 4 wheel drive for the first time.

I'm noticing what I guess you'd call a dull howl when 4x4 hi is engaged. Not sure about low because I didn't try it. Speed doesn't seem to matter.

I figure it's not the tires themselves because I'd hear it all the time.

Just wondering if this is normal because I couldn't hear a thing in the F150. What do you guys think? Should I get it checked out or is that just the way it is?