I have a '99 Explorer Sport 4.0L 5-speed manual 120k miles with original components (tranny, clutch, etc.) and occasinally there's a loud hum (sounds liek a loud "low tone generator")from the engine. I have replaced the alternator recently and replaced the wires, plugs, and coil pack. it happens once and a while, and only happens when i'm going slow, at a stop light, or have it running and not in gear. it is a low tone hum, and i can't figure it out. If it happens, and i start to move, until i break about 2000-2500RPM it still is there, and it's only there in first gear. Once i drive or turn off the engine and let it sit for a few minutes does it go away. it never happens while i'm driving or in gears 2-5. (when i'm in 4x4 low wheeling or at the boat launch it tends to happen too, but the 4x4 works great and the noise sounds like it's coming from the front part of the engine.) Also, there's no performance issues with this hum (i'm not losing any mechanical power and there's no electrical power loss...I just don't know what it is and what is causing it. I thought it could be assicited with the fan, but i already replaced the fan clutch (it was shot and the fan was always engaged). the tranny clutch is working great and no signs of it slipping as i run through the gears. Any thoughts would be great!

 

I have a '99 Sport as well and had a humming noise last year and had the idle control valve changed and that fixed it right up. Apparently a common Ford problem.

 

Thanks for the heads up! how involved was it to change out? how much $$?? Thanks again!

 

Well I had a mechanic change it and I don't believe the part costs more than $70-$75. It was simple change and I was in and out. Sorry I'm not too mechanically inclined to give specifics on the swap out but seemed simple enough to me. Hope that solves your problem.

 

It happened this past weekend, i checked it out and it seemed to be coming from the air intake components. i took out the air filter and checked it and nothing was out of the ordinary, but the noise was louder with the intake not clamped to the filter housing (which makes sense). i also noticed that my coolant tank was bone dry, but i had coolant in my radiator b/c my temp gauge worked like a champ (and was normal). I know i have a small leak but i'm not sure where the coolant goes as i have to add it about every 8 months or so. i added coolant and the hum went away (for now)...any ideas here???