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Any competent mechanic will tell you that it could just as easily be the bearing in the idler pulley and not the belt - especially so if you just changed it. You could give it a squirt of silicon lube (I don't use anything else these days) and see if it quietens down.
Failing that - and if it's not your alternator (please, please let it be something else - alternator isn't cheap) then some "dry lube" on the inside of the fan belt usually shuts them up, but could lead to glossing of the belt later on.