So I know what you are gonna say.. It's the {alternator, idler, tensioner, or other bearing}. Nope this ones different, please read on
Details: Always at or above 40mph
Windows down or windows up
No correlation to RPM
Pitch never changes
Volume does change
Volume does change often when I feel a cross wind
Always stops when decelerating

So for a few months now I've had this noise, it most definitely did get worse since it's been below 10* since yesterday. last night for example was the loudest it'd ever been and while I don't crank the radio like 18 year old teenager loud. I do put it up there and the whistle wasn't drowned out. I could hear it with the window down too. In 4WD, in 2WD.. on gravel and pavement. With a few hundred pounds in the back or without any load. What's odd is that nothings changed on my truck since the e-fan project and that's been complete for ~ 1 month. If this is just wind noise, then why does cold air make it worse?
Today I'm going to zip tie some cardboard across the entirety of the area behind my grill. Block it off totally and see if that affects it. I read somewhere LATE last night about wind noise generated by the crack around the fog light assemblies. I'll cut some foam and block that off too.

Before anyone says that I'm being dumb, and it's the alternator bearings, lol. Both the front and back bearings are brand new in that sucker.. replaced themself with a Timken and a National. This was back during the summer when I goat that 150 amp alt from the JY. I know what bad alt bearings sound like, this is not that.

 

Well, turbos can make noise when there are problems. Do you have turbos? Wind whipping thru the 2-arm tow mirrors can make noise, although usually not too loud. Do you have tow mirrors?

 

I friggin wished I had turbos! Also no.. no tow mirrors. No low-hanging fruit sad to say.

 

Water pump? Do you smell coolant ever?

 

New water pump even.. but no, no coolant smells.

 

Windshield seal? I had my windshield replaced. Found out previous owner went the diy route and replaced the windshield himself (did a horrible job). Old windshield seal was applied insufficiently and had an air leak. Only noticeable at highway speeds.
Cold temps could be causing your seal to shrink thus making the whistling more prominent.

 

you have a bug-shield on the front? my other 2011 F150 had high pitch whistle and to come find out, it was the bug shield , so I removed it and the whistle was gone.

 

So yeah I do have a bug shield, been in here for the life of the truck as fire as I can tell. Have been thinking of taking it off anyway so that’s worth a try.
and the windshield seal.. interesting. I’ve only had the cowl off once. Didn’t notice any thing with the seal but I also didn’t actually inspect it.
i have been wanting to get a go pro, take a drive with it under the good just to see what I can hear.

 

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As for the whistle... Is the HVAC on? If so, does the whistle go away when the HVAC is turned-off?
.

 

Fair enough.. I did edit my sig but here’s the details:
2009 XLT Screw 4x4 4.6l 3V
158k miles.

No AC. So today I did a couple things : filled the tiny gap around the fogs with pipe insulation, blocked the entire grill from the inside, and removed the bug shield. I think it’s better? But definitely still present and under the same circumstances.

i was reminded earlier, the rear passenger door has some rot along the bottom under the sill as it bends underneath the edge of the truck. That side is the worse as the last about 6 inches of the rubber strip is hanging down outside the truck. Now this is the weatherstrip that the bottom lip of the doors press against. I don’t believe there is any hole to the outside down there because the actual weatherstrip for the entire door, you know the part that encompasses the entire openeing is there and complete.