Sounds like tensioner from your description. Loose belt slips and squeals on pulley with most resistance. Probably alt on start-up, then a/c when you activate it.
You could very carefully , and with another's assistance behind the wheel, vary tensioner pressure manually and repeat your test.

98 Mystique LS v6 atx 110,000km
97 Contour LX v6 atx 125,000km
stock CD on LS, cass on LX
spoiler, tinted windows on LS only
4 wheel discs on LX only
All other available options on both