The hissing sems odd, the fact you can even hear something when the pump is running. I have a 12 year old superpump that leaks around the big O-ring gasket in the pump housing every spring when I first start it up. Very slight dribble. I just torque the 4 bolts to 13 ft-lb and it solves the problem for another season. If you can see exactly where the dribble is coming from that would help. Could very well be the shaft seal.
Al