diagnos a noise in running gear?

4x4 GMC 1500 has a "whooping" noise much like a tire loosing a belt might make.  new tires, aligned, bearings and ball joints new in last few hundred miles.  noise increases with speed and is more pronounced in reducing almost like a weight is shifting in running gear.  U joints old but do not seem loose front or back.  wonder if it could be a drive shaft or a bearing in the rearend or something in that direction.  feels like in rear but not absolutely.  vehicle does not vibrate at any speed.  350k miles.  have changed drive shaft and u joints years ago.  this came on gradually about 6 weeks ago and now quite clear but not loud.