It could be either a wheel bearing going bad or the front differential could be low of fluid. I would check the fluid first if it is low it could prove to be expensive if the ring and pinon gear gets burnt up or even the bearings. The plug should be in the driver side of the differential case.
bad wheel bearings will do that, usually in the front.
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
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.
radiator fan.
Unplug it.
The differintial is starting to wear...
check your wheel bearing
Humming noise on the front passenger side of a 1997 Ford Explorer XLT may be due to a bad tire. Try to jack up the tire and spin it by hand to see if the noise is there.
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?
Most likely the humming noise is coming from a contactor located in the outdoor unit if you hear the noise outside or from the air handler or furnance located inside the home,if the hum is inside the home
possibly your running RCAs on the same side as your power cable