05 s80 squealing / high-frequency squeaking
 

resolved, thanks for that tip on the exhaust.

Small leak in the exhaust? Maybe the exhaust has a slightly loose hanger causing it to shake, chirp, or rattle as you go, speed just causing more vibrations.
Just a guess or something to look for, hopefully you have a shop or garage, -20 is a bit cold to just grab the creeper and take a look underneath ^^

Quick fix that could save you a grand later.. might possibly be the idler pulley. The bearings wear out and chirp as they rub against each other. If it breaks on the freeway it will take the s belt and timing belt w it. Also such high mileage may need to do the timing too but definitely consider the tensioner and idler pulleys.