I have a 2000 4x4 with a 5.4 I just installed a K&N FIPK and I noticed it makes a load hissing sound when the engine is warming up and when I let my foot off the gas.
I check all the connection and they are tight, there does not appear to be a vacuum leak. I saw a number of posts about a moaning sound from the 4.6, this it was even mentioned in the K&N installation instruction with a recommenced fix, but nothing was mentioned for the 5.4.
Has anyone else had this problem is the a way to fix it?

 

I have the same setup with no problem . . .

. . . but, I do remember reading somewhere that you can over-tighten the clamp and cause the inlet side of the air tube to kind of bend out of shape and cause a leak.

Did you try to track down exactly where the leak is . . . ?

 

I did not track down exactly where the leak is, but I did tighten those clamp down very hard. How would I find where the leak is, any suggestions?

 

In my experience, that is normal for an "open air" filter. What you hear is the air flowing into the filter. I have the same setup on my 4.6 F150,and I kinda like the sound myself.

If you open the hood, then push/pull on the accelerator cable with your hand, you can hear that the "hissing'' sound is coming directly from the filter.

 

I get the same sound here, only when the engine isn't under any type of torque. I.E. when I take my foot off the accelerator at highway speeds and let the engine brake some. It is a high pitched whistle more than a hiss... It really only seems to happen in gear too, I can't put it in park and get it to make the sound. I dunno, but maybe it is normal? I have a K&N too...

 

FYI:

I sent K&N Technical Support an email and this is what the said, I will give it a try:

From:
John Pacheco
K&N Engineering
Technical Support
tech@knfilters.com

The noise you are hearing is the IAC valve working. Ford V8's have had
whistling, humming, or honking noises emanating from the idle-air bypass
valve for years. Ford usually installs a resonator/muffler to the small
hose that brings air to this valve. A few years back, there was a TSB
circulated for a relief kit that Ford made to cure vehicles making these
weird noises even when totally stock. Apparently this kit modified the
valve itself and redirected the air as it entered the intake manifold.
Obviously, when an FIPK is installed, these noises will be even more
noticeable than before. My suggestion world be to get 27" of 3/4" heater
hose and replace the plastic tube from the intake to the IAC valve. In some
cases that will eliminate the noise. If not, you will need to contact your
local dealer about the IAC relief kit.