it would depend on what kind of humming sound you are referring too. If you are over due on a transmission service or what not it will humm at higher speeds and as you slow down the humming slows with it, or if your brakes have gone bad it will make a humming sound once you hit over 30 mph. it could be a lot of things but those are some options to check out
Check the bearings.
Check 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.
radiator fan.
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
bad wheel bearings
Perhaps the cooling fan is running?
Wheel bearing?
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?
it is the universal joint front or rear have someone outside vehicle listen to where its coming from front or back. front of driveshaft or back of driveshaft to rear end.
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.
Possibly the wheel bearing.