I have a 2017 Ford Flex with the 3.5L NA engine that makes a very loud chirp/squealing noise. It only happens when it’s cold, in motion traveling between 35-65 MPH. The sound it’s self does not happen everytime we drive it, and the chirps are sometimes short quick chirps, other times and a series of long spaced out squeals. Generally happens after moderate acceleration but not always. It never happens at idle, on startup or at low speed. I finally caught the sound on video, attached is a link to a YouTube of the noise, it happens 3 times during the 45 seconds, first quick chirps and then long squeals.

 

Did you ever figure this noise out? My 2014 Taurus makes the same noise when cold.

 

From the little bit I was able to hear , Sounds like a dry bearing noise.
Could be , Tensioner pulley bearing
A/C pulley bearing
Maybe even an alternator bearing

If you can, before starting the engine remove he serp belt and spin each of those pulleys. You might be able to feel some roughness or maybe even hear the noise.