First off - your truck is fire.🔥 Gorgeous 2008 Limited. I'm not usually a fan of the "metallic" type trims, b/c it's plastic, but it looks like the way Ford did this factory metallic trim (lower door side panels, door handles, front grill ) accent very well on your rig. Plus I'm digging the King Ranch side emblems and you found aftermarket wheels that were a great compliment completing the aesthetic. I must've been buried under a rock or in a cave and think i should've snagged another ST in the later generations, at least a preowned before production terminated... But like many diehards in this forum i thought the model would survive another decade.
But back to the point.... I own a 2002 very well maintained for it's age (22yo) with 235K. Prior to swapping out my entire Rear End 2yrs ago to enable installing Rear Disc Brakes for a long desired upgrade. i had this annoying intermittent squeak that i thought would drive me into insanity. Like 2 hampsters that only humped during longer rides solely for the purpose of torturng my ride solitude... Wouldn't do it much if at all at low speed - maybe if i hit a bump, but usually only highway driving speed 50+ or the like. I thought i had loose trim, step rails, or the like. I hunted and hunted the sound and thought it came mid or aft the cabin - but of course i could NEVER replicate it when it took it into a repair shop for brakes, tires, rear diff check, etc. It wasn't until the swapping out the Rear End/Brake upgrade my mechanic (an ASE certified mechanic/family friend/brother in Christ) said "your passenger side axle bearing is shot - I don't know how you've avoided it failing and locking up the wheel. The driver's is okay" - and that's what the root case of the squeak was. Replaced both axle bearings which completely eliminated that squeak. I told him to add extra, extra grease..... But again, after multiple service intervals where no one could ever detect my issue i was ecstatic to have a peaceful ride restored. I can ride and not listen to music or anything, but hearing a noise that shouldn't be there deafening.
I realize a "whistle" isn't the same as a squeak, and the 2nd/3Gen have significant build differences, but just in case.... maybe this could be similar??? If not, simply the encouragement to keep hunting until you find it. 👍 Riding in PEACE is essential.