I am experiencing a similar problem. I have a 2004 explorer, 4.6 v8, 4 dr 2wd. My wife has heard it many times, me only once. I have read there is a problem with the rear bearings in the form of a TSB. Does anyone have info on this? Is it serious enough to have to replace the bearings? Or can I continue to drive it as is. I have suspected the AC but my wife said she turned it off and the noise continued. HELP! This is driving me nuts.