whistle noise in 98 ford taurus

I have a 98 ford taurus it periodically has a whistle noise in the engine compartment nothing seems to make it better or worse. I was told my alternator was over charging so I replaced it. BUTTTT it still
does it but not all the time. any suggestions
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Do not listen to the person that told you to replace the alternator, would be one good suggestion ;)
mike

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I already changed that any suggestions on what it could be????????????????
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