Could be the differential whine that's plaguing a lot of people. Does it just happen recently? Or is it under light throttle?
A couple of things too, it can be tire noise (the fact that it only happens at certain vehicle speed, not engine speed triggered this suspicion), or it can be the famous "cold air intake" whine.
Try this: Rotate your tires and see if that elimiates the noise. If so, then it's tire wear. If not, it may be the differential wear. Try a new air filter as well, and if that alleviates (but not completely eliminate) the noise then it's the valve behind the mass air sensor that's causing it when air is rushing through the valve.
Otherwise it's the differential whine. Search here and .org for the TSB number and bring it to your dealer's attention.