Although the Ford service department "fixed" this problem by rebuilding the entire rear end, the same problem has surfaced again at exactly one year later! They have offered no explanation as to why this happened (they report there is no leak) only to say I brought it in 5 days after the warranty (I had tires balanced & rotated, other things fixed, never dreaming the same problem was the culprit, until the humming noise became obvious!) and that is what warranties are for. So, I am left to wonder, WHY has this problem recurred, and so quickly? Is it an issue with the vehicle, or with the service that was done to it? Apparently the question to the Ford powers that be is of little importance, it has been 2 weeks since I took it back in. I have gone to other shops to confirm the problem, and inquire about their service standards, all of which seem to offer better service guarantees. Ford will not touch this vehicle again, and although this is my 3rd Ford to own, I will consider other makes for future purchases.