squeeking metal on metal sound

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The belt wheel below my alternator is making terrible sounds.. I noticed is is moving instead of staying in place. It looks loose... I had what my harmonic balancer replaced about 4 months ago on the advice of the machanic.. I thought my beld had gotten a little loose but they said it was the balancer... The car seens to run fine except for the noise.. I had one guy tell me my alternator might have to be replaced, but its the wheel below the alternator.. Help.... Whats happening.. I don't know who to trust.. greg classicprop_gg@msn.com