My brother and I just installed the AF1 on my 2004 f150 4.6. I can feel the increased power and I like the "growl" when I get on it. Heck, I can even break the back tires loose now and I'm running 285/50/20's. Now for my question. I can hear the air being sucked in (whistling sound) between about 2200 RPM's to about 2800 RPM's. Is that normal? It almost sounds like a belt slipping.

 

I've got the K&N intake and I can also hear a whistling if I have just the right throttle position. I believe this is normal and the plus of the HP and sound outweigh the minus of the small whistle I hear once in a while.