2003 Oldsmobile Silhouette Buzzing,

At speeds over about 10 MPH, this sound begins in the rear of the vehicle. The buzzing increases frequency with speed. Speed seems to be the only thing that affects it. Acceleration, braking, turning, even coasting in neutral, all have no effect on the noise. I had the wife drive me around the block while I rode in the back and pressed my ear to the floorboard. I couldn't tell if the sound was coming from one side or the other, or both. I put the rear of the van up on jacks this morning, took the back tires off and inspected a bit. I didn't notice any visible problems with the wheels or drivetrain components. I spun both wheels by hand and they felt normal, with no out of the ordinary sound or resistance. That being said, I don't have much automotive experience. I do my own oil changes and brakes and am comfortable swapping alternators, water pumps, changing belts and hoses, etc. Past that simple stuff, I'm in over my head. What should I be looking for in this instance? Thanks!