I'm not very experienced with Explorers, but from my experience with other vehicles, the noise can be caused by a bearing problem, like the front differential going out. However, I had a similar noise on my Grand Cherokee and it ended up just being the front swaybars needing replacement from having a 3" suspension lift. Anyway, does the noise change with an increase in speed at all or when gradually slowing down? How many miles are on your Explorer? Good luck finding a solution to the problem.