[ENINTY]

It is most likely a bearing in one of the pulleys. The belt would only squeal if the tension was wrong. A classic belt squeal comes from the belt slipping on the surface of one of the pulleys rather than rotating the pulley, which only occurs if the belt tension is incorrect (i.e. loose). The tension is constantly and automatically set by the belt tensioner because the tensioner is free-floating and the tension is set by the coiled spring in the tensioner arm. So the squeal is either the tensioner is bad because the load of the spring is not being properly transferred to the belt (this is usually not the case), or the bearing in the pulley on the tensioner arm is going bad, or the bearing in the idler pulley is going bad. It squeals when it is cold (first started up) because the bearing has set overnight and is cold and smaller in dimension than when it warms up, which lets the ball bearings inside the (sealed) bearing races slide around rather than rotate between the races, or the bearing is temporarily frozen (or slow) and belt is sliding on the pulley surface until the pulley starts to rotate at the proper speed. Once the bearing warms up a bit the dimensions of it change and it starts working almost normally.

It is easy to check for a bad pulley bearing. Just remove the belt and free-spin both the tensioner pulley and the idler pulley. If either does not spin freely and smoothly, and is "grainy" (like there is sand in it), then the bearing is going bad.