humming noise

Hi guys, I have posted this topic many times and still my problem persists so any additional info will be greatly aprechated, i have a defender 90 200tdi convesion, when driving at approx 40-50 mph and I let off the accelerator I get a slight vibration and a humming noise for just a few seconds, i get it when I start to accelerate but again only for a few seconds, right the bits I have replaced, rear prop (with new uj's),rear axle (disco 300tdi one) rear trailing arms, the fault has been there before I replaced all of this, I have had centre of seat box out and don't seem to be gear/transfer box and seems to come from rear of vehicle, swapped front tyres to rear but still remains, the fault is intermittent sometimes really bad and sometimes sweet as a nut, i noticed this when I put 235/85r16 insa turbos on and 2 inch lift blocks, what could it be, im going insane now. Lol

Wheel bearing possibly

Diff bearings, or perhaps more likely pinion bearings, when easing the throttle you are making the axle the power unit, therefore the crow wheel becomes the driver and the pinion the driven.