I have same sound in a soprt ´99 but only at a red light when have de A/C on for a while, idle is ok but sound is awful!!

 

I have sort of the same problem. I have an eratic idle but no noise and no check engine light. I have not worried about it so oh well.

 

My 00 Sport has the same noise. It is more pronounced when I let off the accelerator, and then back on. I suspected it was the IAC. I'll wait until it gives more problems than just the noise, and then replace it.

I've seen so many posts on this same howling, it's almost like this is a "normal" noise for these models......

 

I had the same problem on my 1999 Explorer 4.0 and they replaced the IAC valve. That was last year and the noise is back again. What could be the problem now?

 

Wow...I'm glad I happened across this post, b/c I have the EXACT same problem on my 96 2WD Explorer XLT w/ 4.0. My problem was that whenever I accelerated from a stop or from an idle-crawl on the freeway, it would moan briefly. I.E., I press the gas at a stoplight, and it moans until I get some speed up. Very occasionally, it would degenerate into a constant howl until the vehicle cooled off.
Looks like others have taken the same route I did. It was loudest at the air cleaner, but when I removed the air cleaner, the noise continued. So I figured it must be in the throttle body. One time it was moaning like hell so I got mad and struck the IAC valve with a 2 X 4 (domestic violence, I know!). The noise disappeared! Playing around with it further, I discovered that when it was howling, I could control the noise by loosening the IAC valve mounting bolts. When it was very loose, the car would idle fine w/o any noise, but when torqued to spec, it would howl.
I tried cleaning the valve, but it didn't help. So I replaced the IAC vavle b/c Ford IAC valves seem to suck in general (3 replaced on 3 Taurus-es as well...My fave joke is that the only part that stranded me on this American car was the part made in JAPAN!!!) The howling noise has not come back, but the acceleration moan is still around, in much reduced form. I can live with it.
I'm not sure what's going on here, but I suspect some sort of resonance. My sister's 99 Contour has the same exact problem. Looks like Ford has had problems with the IAC vavle in some form for 15 years, but hey...Warranties don't last that long, so what do they care, right?