Perhaps a bad bearing in one of the belt driven components i.e. water pump,belt tensioner, alternator,idler pulley?
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
Check the bearings.
Check the wheel bearing.
Perhaps the cooling fan is running?
My first guess is that you are hearing the fans coming on. They are doing their job, keeping your engine cool. On a real hot day they might even run after you stop .
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
it would most likely be the bearings but in some cases sometimes the axle make a humming noise when traveling at high speeds.(front wheel drive)
you need a new heater core
Worn passenger side front wheel bearings.
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.
the Parthenon has the columns going all around the whole thing and the Lincoln memorial just has them in the front.