it still could be the belt. i've had new, properly tightened belts intermittently squeal before.

maybe the belt or pulley gets glazed or something...i'm not sure...but i've had good luck with CRC belt conditioner to shut them up.

anyway, it's easy to check for sure if you have a mechanics stetheocope (if you don't, i recommend getting one...they cost less than $10 and are a great diagnostic tool).

just put the tip of the stethoscope on the bolt that holds the pulley on. you can hear the bearing very well that way...if it's got problems, you'll know it.

also, make sure you don't overtighten the belts...that will trash the bearing and put undue stress on your crankshaft. definitely better to err on the side of too loose, imho.