The cause is most likely a worn out wheel bearing.
Wheel bearing?
Check the bearings.
Check the wheel bearing.
An abnormal humming could be emitted from the alternator, water pump, the a/c pump, any number of things.
front axle actuator not disengaging probally due to rotted vacume line across crossmember
I had one that had ring and pinion wear causing the problem could be the axle u-joints or wheel bearing assembly. With typical milage for your year you should be ready for either.
Where is the humming located front, rear? Could be a vaccuum leak if high pitched, wheel bearings, improper brake installation. More info needed.
Check the tires for cupping. If you have had an alignment problem the tires could be cupping and will make quite a lot of noise. It could also be a wheel bearing.
Could be many things but a front wheel bearing/hub assembly going bad comes to mind. Scalloped worn tires can give you a humming noise also.
1999 Mercury front and humming noises can because by several things. Tire noises are the most common cause. A tire that is not wearing properly can cause a humming noise.
is the humming coming from the front end of the vehicle? i have a 1996 explorer with the same issue. yes the noise come from the front but not all the time seems to make the humming noise when it gets hot
Possibly a wheel bearing