i have an 08 4.2 with an intake made for an 04 heritage f150 4.2L. had to rig it to work but it also whistles but only like 2500-4500rpms and at about half throttle. and if you suddenly hit the gas you hear a short whistle like a chirp.

has it been cleaned lately? after i clean mine on my 4.2L it sounds different. you can hear a loud sucking noise till it gets a little dirt on it, then it whistles more.

my 04 4.2L which my intake was made for, sealed to the hood with the heat shield and i had no whistle, its now on my 08 4.2L and it whistles but i couldnt use the heat shield on it since the fenders are wider on my 08 than the 04 heritage was.

 

You know, my cheap K&N kit makes almost no noise at all. I kind of would like to of had a little more throaty sound from it, but it doesn't squeal or whistle either.

 

mine definitely sounds more throaty. actaully on both the 04 and my 08. but the 08 sounds a lot better than the 04 did IMO. the 08 has a bigger diameter exhaust, its a 2.75" and the 04 i believe was a 2.5"