Could wheel hub be cause of noise

My 1997 Ford Taurus moans after I drive it 75 miles up the interstate and then slow down in city traffic. Cannot find a relation to speed, brakes, turning, etc. My mechanic said it might be the hubs, but couldn’t remember if mine are sealed or not.

I spoke with the folks at Ford today. They are totally baffled as well. Any suggestions would be greatly appreciated. The noise comes from the middle of the back seat.