Possibly the wheel bearing.
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.
Answer Humming noise from front endI just got my car fixed with the same problem and it was the wheel bearing and it is gone. Was it a fwd car?
Certain types of tires may cause a brief humming noise after your initial acceleration. The transmission may also cause a brief humming noise.
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.
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
i would first balence the tires then try replacing the wheel bearings if the noise is coming from a specific area on the car In example left front, right front etc.
when the pump is going out it will make a humming noise from the bearings even if the resivore is full.
Check the bearings.
Check the wheel bearing.
A failing wheel bearing is a likely cause.
Bad wheel bearing
radiator fan.