Have you tried running it for a few minutes without the belt to eliminate any of the other pulleys and eliminate the belt(or improper tension) from being the noise?
Does only when your driving or does it do it while stopped too.
It'd be better if you could just post a video of the noise. Save a ton of back and forth random questions and answers. Maybe get a shot of the top of the belt from the alternator to the crank too if you can(with the ac on and engine running{obviously}).
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)