2000 Ford Expedition

Could be hubs or wheels bearings. I lean towards the hubs due to the fact wheel bearing usually hum or growl.
Another area you might want to check first is the shocks where they mount to be sure all rubber noise reducers are in good condition the stabilizer bushings could also make this sort of noise without hearing it personally this is the best suggestion and most frequest wear parts that tend to cause these noises.

Does the condition occur during normal flat surface driving or while driving over bumpy roads?