Have had a whirring noise for the last couple weeks that increases and decreases with speed. Has seemed to be getting a little louder over time also. 2003 4x4, 135,000 on her. Just checked and the 4 wheel drive is engaging and disengaging o.k. Thought maybe it wasn't disengaging, the noise is similar to when it is in 4 high and expressway driving. Any ideas? Does not shimmy thru the steering wheel, does not pull at all while driving.