I have a 97 f150 with 102K on it and lately I'm hearing a "WOOOOOO" noise that comes in around 1700rpm and then fades. I don't detect the noise when it's first started up but after a few miles I will hear it when the engine reaches that rpm range. It will do it in neutral also so I don't think it's trans related.....sounds like a vacuum leak. I pulled the throttle body of a few weeks ago to clean the EGR and at that time checked every vacuum line and fitting I could find......found no problem's. Sometimes I swear it's coming from under the dash but it's really hard to tell.....Anyone have a hint or suggestion I would very much appreciate it. Thanks, Scott.

 

Could be your air filter. If you hear it in neutral, try removing your air box and air filter and see if you can reproduce the noise (while in neutral). Wouldn't recommend driving down the highway without a filter.

 

The iac can cause this type of noise also. Try tapping on it as the noise is happening and see if it goes away. If it does, replace the valve.

Robbie