There is a sensor on the bottom side of the pump that can vibrate loose. This has happened to me before. I have replaced my pump well over 4 times trying to find one that does not whine with my big tires + wheels :-( haha
Can you tell if it is leaking from the pump, the reservoir or the hoses? When my Trooper's PS blew, it all came out at once and I lost power steering right then and there....
Bart