Slick...the only thing I can think of is that the notched washer was not fitting all the way into the shaft splines. So the washer is butted up against it, the little ditty still gets torqued to 100ft/lbs and the washer gets dog eared over, though because it is not in the splines it will rotate with the nut.
Pop the cover on and take that washer and nut off and have a look see....might be a real easy fix. No sense fretting until you have a second look.
The squealing you are hearing might be the pulley moving in and out abit since there is now slack on the spline and the belt is rubbing on the cover.
.....and if I missed someone suggesting that already....just ignore the comment.
D