My truck sound like a leaking baloon..
 

If you have ever heard what air being leaked out of a balloon slowly by pinching the end you will know the sound I am going to talk about. It's a high pitched squeal/whistling sound that pitches with the rpms. As I slow to a stoplight It will start high and pitch lower as I go slower until I stop. If I tap the gas it immediately goes away. Never on acceleration do I hear it. Once it started while in park at the pharmacy drive thru but it usually starts around 30 mph down to about 5 mph before it goes away. It's intermittent but becoming more frequent. I have a Roush blower installed and am hoping it's not the rotors making the noise. It's not real loud either. It only happens when vehicle is at operating temps. I'm thinking vacuum leak somewhere but I could use some ideas from the collective.

/Does no one have any clue as what this sound might be

You can try rolling to a stop by putting it in neutral and see if it makes noises.

I am guessing, take a good look at the entire PCV system. Start with the PCV valve.

Probably something in the supercharger system.

Putting in N didn't change the noise. I did notice I was low on Prestone so I topped off the degas bottle and that made the noise less pronounced. The PCV on the 5.4 doesn't need maintenance. The hoses are all good. Once again it only happens when I'm running high speed and slowing down. It doesn't make a peep driving around town. My guess now is that at higher speeds the temp goes up and puts the most pressure on the cooling system. If my degas cap is bad that might explain the timing of the noises which sounds like a tea kettle. The blower is tuned to engage starting at 25% throttle so if it were the rotors I would hear it driving locally. Pretty much only getting off freeway and a mile or two after until it cools down a bit.

It almost leads me to believe it might be something radiator or hose related. Have you checked your radiator cap?

UPDATE...the noise is still there and getting worse. I have now changed my thinking to the Roush blower intake gasket or an exhaust leak. Problem is Roush has discontinued the gasket that mates the blower to the intake man. WTF are they thinking? I don't know how many 04-08 F150's have a Roushcharger on them but it seems like the gasket kit should be available even after 10 years. My truck is an 05 and I can get almost anything for it so Roush needs to get with the program.

Now I have to use goo-sket instead of the metal one from Roush. A guy on the phone told me to use Permatex Black RTV...anyone else in this boat? Are there different p-tex blacks? Are there metal cut-to-size gaskets out there? I know there are paper gaskets you can carve like a pumpkin but metal....dunno.

Any and all feedback on this is always welcome and thank you in advance for those who do.

I would recommend going to your local metal fabricator and most likely they have a laser cutting system or high pressure water jet. They can measure up the old gasket or give them the part, and they can make a gasket for it out of metal.