Got It!
I found the problem here, and it's disturbing to say the least. Always check the last thing you've worked on! For this hamburger mechanic, it was the front brakes, all new Motorcraft parts. Thing is, upon another inspection behind the wheel I found the top caliper bracket bolts on both sides were loose, as in several threads showing. I was hanging on these with a 24" breaker bar! Turns out, the threadlocker that Ford applies at the factory isn't just an over-precaution. I loaded them up with Loctite Blue and torqued them down. Noise solved! I guess when they warmed up, the bracket was moving enough to drag on the rotor a little, causing the whistle noise. Wish I had found this before I towed a 5500lb boat 500 miles home from L.A. Hope you guys can find something useful from my mistake! Thanks again for all the replies.
