Tires? Rotate lately? If theres vibration when applying brakes ....Rotors. Bearings are usually a constant noise.
+1. Had a wheel bearing go out on a previous truck, was a constant noise. Noises can vary, of course, but thats the usual.
Tires could be out of balance as well, lost some weights or something. I've had tires in the past that, when badly out of balance, made that noise.
When mine made that noise on stopping, it was because one of the caliper bolts had broken. Rear caliper was grinding against the inside of the rim by one bolt, and it made that noise only when stopping. However, I'm assuming you'd have noticed a loose caliper/pads.
I had just changed the brakes a few hundred miles before, and I know I tightened all the bolts. Not sure what happened. New bolt and she was good to go. Unfortunately, the parts stores (Advance, O'Reillys, etc.) had the wrong size bolts, and I had to get one from a hardware store, but that's another story.