IAC
This is something I might be able to help you with:
I bought a 1999 XL last Sept w/ 70K. I am not familiar with new cars since I owned an 84 Ford LTD as original owner. Best vehicle I ever owned, but for some reason I wanted a change after 200K.
Anyway, mine was making a howling noise that I really noticed while driving into the garage, and I am pretty sure the A/C was on since it was hot out, but I don't think it mattered wether or not it was on or off. Almost sounded like a loud honing noise, and I was afraid a bearing was going or something serious. I took it to the dealer and he listened for 5 seconds and knew right what it was, the IAC. Well, I had them do it for $190, but that was because I felt as though they were the one's to diagnose it, and I really was too timid (not now) fooling around with such a sophisticated system that was so new to me/////
Hope this helps ( I think a new valve is around $90 from Ford and can probably be changed in 5 minutes)
Hope this helps, and it's the same noise I am refering to