I'm hoping someone might give me a clue here. I have a 1998 Ranger Extended Cab 4.0 V6. She has about 171,000 miles on her. Still runn'n good but I have a whistling noise that is driving me crazy. It only makes this noise when the truck in moving forward. If it's sitting still and I rev the motor it does NOT make the noise. The noise is constant when the truck is in motion. It's not just when I apply my brakes. My brakes are pretty new. I had this problem about a year ago and replaced the IAC and the noise stopped. When the noise returned I replaced the IAC again but this time it didn't fix the problem. I've also taken out the air filter and had a look in the air box. Everything there looks normal. My truck is all stock. I have never tried to mod anything. I'm hoping someone might give me some ideas to look into.