Is there a way to tighten and idler pulley for the serpentine belt.

Tensioners will allow the fan to be turned easily in the direction of rotation when the fan hub is locked, but you shouldn't be able to turn it backwards easily. Make sure all tensioners and idler bearings are good. Also, grooved pulleys can get rubber build up in the grooves that causes a squealing. With the belts off, a pocket knife in each groove and spinning the pulley will peel it out. It's hard to see, so I try a few grooves every time I change a belt. Anything pointy and rigid can be used to clean them out, a wire brush doesn't work so good. You can't spin the crank pulley, so that's a little harder to clean.

If all looks good and you really want to tighten the belt, the easiest way to do that would be to take you belt to a parts store and ask for one 1/2" shorter.

 

Be sure you have the correct length belt, but parts people never make mistakes.

 

Belt dressing might help too. A little goes a long way. As stated above try cleaning everything first. If the new tensioner has paint on it try scuffing the paint with some fine sandpaper.