Product review (4 stars): Bad Idler Pulley Bearing Caused the Noise
My Amana (made by Whirlpool) dryer is only about three years old. The only part that was significantly worn was the idler pulley. The sintered bronze bearing ran dry. The surface of the bearing was rough and caused it to seize on the shaft. This created the squealing noise as the belt was rubbing against the face of the pulley because the pulley was not rotating.  It could be a manufacturing defect such as failure to impregnate the sintered bronze with oil after manufacture or a defect in the sintered bronze itself. My last "Whirlpool" dryer had a similar pulley and it never failed in over ten years of use. The new pulley fixed the problem but how long will it last?