alternater produces loud high pitched sound, short life

I would like to find what is causing my alternators to live such short lives. With in months of putting in each rebuilt alternator the alt. will start to produce a high pitched wine. It starts quietly and gets worse. It also continues after the truck is turned off. After the engine is turned off the wine will drop in pitch. in looking at the diodes after the failure they are cracked .

I have checked the following things and ruled them out. worn serpentine belt, bearings on alternator and idler arm. Loose intake manifold. I have had the alternators and the electrical systems tested by three garages and no one can find the cause.

I have had someone suggest the alternator gets too hot because of the way it sets on top of the engine. Seems like it would not get a lot of cooling.