Most interesting thought! I'll take a bit of exception to it, though, as I've had 3 Explorers now, and none had any gear noise at all....perfectly quiet. So, if aluminum housings and steel gears and supports CANNOT work quietly, why did mine, batting 1000?
I suspect those involved in most of these failed rebuilds actually do not adhere to the strict instructions given by the Ford Shop Manual. Beyond that, the theory goes, FWIW, that hypoid gears, which all these units have, are pretty unique in being able to transmit lots of power for their size, and do it absolutely quietly, when properly set-up. The difference is that the teeth of hypoids "roll" over one-another, with almost no rubbing friction, almost like ball bearings as a comparison. Further, they have much more tooth area in contact at any given time than plain beveled (angled) gears do. The two key elements to success are absolutely no "play" in support bearings, and close attention to tooth contact pattern as checked by using a colored-material, usually Prussian Blue, which is applied to the tooth faces of one or the other of the gears, then rotating them by hand while maintaining drag load using hand-force. If the contact pattern is properly established, those gears WILL run quietly, so long as bearing preloads are adequate.
I say these things because I have successfully set-up some several hundred gearsets over the past 50+ years. But, rest assured, early on, I fu??ed up a number of them and learned a lot the hard way. imp
