Check the idle air control valve (IAC valve). If dirty clean or replace.
Also check air/smog pump. Be sure intake fins on that are clean. Even if clean, could be causing the noise.
idle air control valve
wheel bearing
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
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.
98 Ford E350 The AIT sensor is just after the air filter housing. It has two wire going to it, one is gray and the other is gray with a red strip. The sensor can be remove easily as it is just snug press into the rubber air intake housing.Read more: Where_is_the_IAT_Intake_air_temperature_sensor_on_a_Ford_Econoline_E-350
Check and see if your power steering fluid level is low.
http://wiki.answers.com/Q/Where_can_you_find_the_wiring_diagram_for_a_1982_ford_e3501982 ford e350
The Ford Explorer uses an idle speed control valve that allows air to flow thru the idle air passage inside the intake manifold. When this air passage gets clogged up with carbon or grime it restricts the passage & the result is a loud humming / moaning noise. The repair is to remove idle speed control valve & spray carburetor cleaner or intake cleaner thru air passages inside intake manifold & clean or replace idle speed control valve.
how many o2 sensors on a ford e350 van 2003
have seen in the past , fuel pump moan/noise on this make & model / from a ford tech.
My 1997 e350 has 6x8
Possibly the EGR system ... try squeezing the hose from the air intake (plastic thing that says SOHC on it) that goes to the rear of the engine (about 1" dia.). If it stops, the humming might be a sticky EGR valve.