I have an 03 mountaineer with the 4.0 V6. The intake is great and sounds awesome; I have NO driveability problems. The only issue is that at exactly 2000rpm I hear a whirrling noise that sounds like vacuum. there is only one vacuum line that goes to this intake and it is tight and not cracked or damaged. It's not really a problem, its just kinda annoying. This seems to be my only issue with the intake, I dont see any other vacuum lines that are damaged or unplugged, and I have plenty of power and better MPG with this unit installed. Anybody got any input or advice???

 

Are you sure it's coming from the intake and not the exhaust? if so, check the joints to make sure they're good and tight. The noise may be the box vibrating (air getting around the seems).