Over New Years I had to engage the full-time 4x4 button on my 2003 Explorer. Shortly thereafter I started noticing an airplane propeller noise that comes and goes with acceleration. It’s pretty loud. It has not affected mileage or dashboard engine indicators. Tires still have 40% left, not them. Have I ruined an axle or something? There is a rear end whine that Ford says is endemic to this model and year, but I think they are not related.
The only thing I can think of that would be related to the 4wd system is the auto-locking hubs aren’t disengaging. You need to get this checked. This could be serious.