Check all your belts and pulleys. Take a mirror and check the underside of the belts or just flip them back enough to see the back. The pulleys should not move.

Does it squeal with the A/C on/off or does it not matter? Does this squeal IMMEDIATELY after you let off of the gas? Does resume while doing 80km as soon as you step on the gas?

 

I've also had the same noise for the year i've owned my ride.

Exactly as you describe it when going over 80 kmh
Hasn't had any impact in feel and driveability for me.
Also hasn't gotten worse or better all the time i've had it.

Btw i've uninstalled my a/c compressor (cold enough in iceland to not use it)
and still have that sound if that helps you guys in problem solving