Possibly worn wheel bearings can create a humming or groaning sound.
the bearings
bad wheel bearings
The motor which blows air through the vents.
One would really have to hear it in order to determine for certain. Are there any particular conditions existing in which the humming occurs? Wheel bearings are a possibility.
If it sounds like its coming from inside the car its probably the fuel pump if not its kind of hard to say.
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
on cars that run with fuel injection it would be the fuel pump, or high performance cars that use electric fuel pumps.
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.
Check the fan belt; it may need to be tightened.
The caliper may be hanging and causing the humming sound. I cant type the sound out but it mine made a humming noise as the wheels turned. If I pressed the brakes hard and quickly let go it would stop. I just replaced rotors and brake pads and it is back to normal now.
The tires themselves may be the cause of your concern. If the tires have deep treads on them they will make some a strange humming sound. I would still advise having a mechanic check it out just to be on the safe side.