Ford Explorer 1997 60k 4.0l auto, huge noise under power

Hi all,

Just got the above car and its driving me nuts.
Engine works fine, so does transmission and 4x4 bits.
There are no unusuall issues apart from the fact that above 40 km per hour and all the way to 100 the car winnes, like a tranny winne or dif winne. Very loud. Take of power and goes silent, put on and back it comes. Hot or cold makes no diference to loudness, nor does go around a bend etc. There is no play in the diffs to speak off, no warning lights, tranny fluid is clean and does not smell burned. My mechanic claims its 'normal' as these cars are very truck like but I do not think so. It is too loud for it. G box works well, downshifts, upshifts, all smooth, any ideas would be very welcome.
Regards, Antonio