I would suspect a bearing failing in either the water pump or generator. Take the belt off and turn the water pump and generator by hand to see if either one is hard to turn or is wobbly.
One other thing to think about, and this might be far fetched, but could the wiring to the generator be mixed up causing it to put extra stress on the belt while running? Seems to me if I remember right you can make a generator run if the wiring is correct to it. Could it be trying to do that and fighting against the belt?
I can't see any way turning right or left would have any effect on the belt and generator.
Perhaps the regulator is not shutting the generator down when the battery is charged.
