jet an other problem, what i can not determine where it comes from.

I replaced both rear wheel bearings, changed the diff oil, inspected the gear (they are optimal), and overhauled the the rear-brakes (OEM) with new parts.

Still there is a hollow kind of noise coming from the back, and I can not find the source.
So i readjusted all the parts from the rear axle once again, but the sound is still there. It has to come from the axle, because the frequency raises and lowers with the speed of the rig.

any suggestions ?? where to look ??