Hey.

I have a 1998 volvo v70 with 150000 miles on it. It is the awd model too. I noticed recently when I drive that around 2200 rpm in 4th gear at any speed over 40 (hell of a combo?) i get a weird groan thats not really that loud...but definately there. Its not always there, sometimes i can have that combo with no noise. I would think its the drive shaft but i had that replaced 7000 miles ago. My dad thinks
it could be a constant velocity arm, but i think its a moving parts prob. Maybe transmission? transfer cas? turbo? Also, I am getting an oil leak down by the turbo charger, it looks like its coming from an oil line going to the turbo. Any help is awesome. Thanks

 

The noise might be the Viscous coupler.
Might be the Driveshaft
Might be a wheel bearing.

The oil leak might be the turbo return line o-ring.