I installed a K&N filter in July in my X (5.4). Due to heat, max AC running etc, I never saw any imnprovments. Now it's cooled off, quieter in the truck now, still no mpg improvment (still 13ish on the display) but notice a slight "whineing" noise that I've never noticed before, in slight acceleration (say from school zone 25 to 45). So I awapped out the old paper filter for a day or 2, still sorta have the same whine noise, but quiter, and doesn't start till about 2.5k on the tach, where before I wan't trying pin point it, but would guess it would start at abou 2k.
Is this normal? Is it just the sound of the engine getting "better" air (less turbulent)?

Thanks
Bryan

 

If its only the filter, and not a CAI system, then its probably just your IAC valve going bad.