loud howling in airbox

[Thepizzacowboy]

I recently purchased a used 99 ranger xlt 4x4 with the 4.0 ohv. It has about 145k. Several times while driving a loud howling noise has started coming from the airbox. It usually only lasts for about 10-20 seconds and seems to begin while at idle or slowing down and stop after accelerating. Today for example it started once while I was slowing down in traffic then stopped as I turned and accelerated up a hill. Then it happened again later in the day after I started the truck. I backed out of a parking spot then accelerated forward and it immediately stopped.

I know it's coming from the air in because one time it began right as I was pulling up in front of my house. I put the truck in park and popped the hood and it was obviously coming from the airbox. It was vibrating but not shaking and the sound was a loud kind of howling/screeching sound.

I've changed the air filter and just yesterday I did the "airbox mod" and removed the air silencer but I'd like to stress the fact that it happened on more than one occasion BEFORE I ever touched the airbox to change the filter, remove the silencer, etc.

Thanks in advance for any ideas or suggestions you might have.

 

[adsm08]

Check the duct work before the box for anything that shouldn't be there.

 

[downer]

mine has started doing this too at idle or low rpm, fairly loud. i think it's just the air box resonating. if you give it a little gas does it go away?

 

[rangerronny]

Check the idle air control valve. Mine does the same intermittently and idles up when it happens. The valve can stick open allowing air to bypass it and you can hear it in the air filter box. The valve is mounted with two 10mm bolts to the top/side of the intake.