My explorer is a 2001 with about 110,000 miles on it. I put it in 4-wheel drive and I am hearing a whinning noise, but if its not in 4-wheel drive then theirs no noise. It has a 4.0 L engine, I have checked the wheel bearings...nothing wrong with them, I made sure the front unit has plenty of trany fluid and the noise is coming from the front...so I don't know what else it can be